My Goals

  • Quick shortbread base

  • Chewy caramel filling

  • Smooth, firm chocolate top

I ’ll concede that millionaire’s shortbread is a corny name, but it fits this impressively rich British cookie/confectionery hybrid: a crunchy, buttery shortbread base topped with a chewy, caramel-like layer, which is in turn topped with a thin layer of shiny, snappy chocolate. It’s an indulgent combination of textures and flavors, all stacked in a sleek package.

I ate a lot of it when I lived in the United Kingdom, and since it has a good shelf life (more than a week), I figured it would make an ideal holiday gift. But after giving a few recipes a try, I realized that a gift-worthy recipe wasn’t going to just come to me wrapped neatly with a bow.

The shortbread base in most recipes was easy enough in terms of composition—just flour, butter, sugar, and salt—but they differed in terms of procedure, calling for softened butter and a stand mixer, cold butter and a food processor, or getting in there and rubbing everything together with your fingers as you would for pie dough. The caramel portion of this cookie is unique: It’s based on sweetened condensed milk, which gives it a luxurious creaminess. Instructions were all simple—cook sweetened condensed milk, butter, sugar, and corn syrup in a saucepan until thickened—yet unpredictable. In some batches the butter separated out, but in other batches it didn’t, and I wasn’t entirely sure why. As for the chocolate, you can’t just melt it any which way and expect it to reset with its original sheen and snap. With each batch, I was left with dull, soft chocolate that was a mess as soon as I touched it. I was determined to devise strategies that delivered three perfect layers every time, and I would streamline as much as possible along the way.

Beta Testing

Knowing that the other two layers were going to be challenging, I gave myself a break with the shortbread and turned to an approach I’ve used before that skips the solid butter. I whisked together 2½ cups of flour, ½ cup of sugar, and ¾ teaspoon of salt and stirred in 2 sticks of melted butter. I patted the dough into a foil-lined baking pan, pricked it all over with a fork to prevent pockets of steam from building up underneath, and then baked it until it was golden brown and firm. My melted butter shortcut produced a level, crunchy shortbread layer in no time flat. On to the chocolate.

Cocoa butter can solidify into any of six different crystal formations, but only one—beta crystals—sets up dense and shiny.

It would be so handy if chocolate were like wax: You could melt it and cool it, and it would always have that same texture and appearance when it resolidified. Sadly, it’s not that forgiving. Good chocolate right out of the package has a nice snap and sheen. But when you heat it past 94 degrees and leave it to resolidify, it takes on a dull, dusty appearance and soft texture. This is because the crystal structure of the cocoa butter fats in the chocolate has changed. Cocoa butter can solidify into any of six different crystal formations, but only one—beta crystals—sets up dense and shiny. For the layer of chocolate for my millionaire’s shortbread, there was no settling for anything but beta.

Since heating chocolate beyond 94 degrees destroys the beta crystal structure, a chocolatier would temper the chocolate to reestablish that beta structure. It’s an elaborate process of melting chocolate, cooling it, warming it, holding it at that temperature for a while, and then warming it a bit more (but not too much!). Luckily, I came up with an alternative that’s much easier and more foolproof: Melt a portion of the chocolate very gently, being careful not to let it get too warm, and then stir in the remaining chocolate, which has been finely grated. These small flakes disperse throughout the melted chocolate, and their temperature remains so low that most of their beta crystals remain intact, triggering the formation of new crystals as the chocolate cools so it sets up perfectly shiny and snappy.

We wanted a chocolate layer that had a glistening sheen and nice snap. To achieve this, we found an easier way to temper chocolate.

I melted 6 ounces of finely chopped chocolate in the microwave at 50 percent power (small pieces would melt more quickly and help minimize the chance of overheating). I stirred the chocolate every 15 seconds, holding the bowl in the palm of my hand so I could monitor the temperature: If it felt warmer than my hand, I stirred until it cooled down before returning it to the microwave. Once it was all melted and still a bit warm, I deployed my secret weapon: 2 ounces of finely grated chocolate. Finely grating the chocolate would ensure even dispersal and mean it needed minimal, if any, additional heating to fully melt; plus, it seeded the melted chocolate with beta crystals. I stirred the grated chocolate into the bowl, heating for only 5 seconds at a time if necessary. I poured it onto the caramel (a placeholder recipe for now) and spread it into an even layer. It started to set right away—a hallmark of beta chocolate. With top and bottom squared away, I moved on to the middle: the caramel.

Breaking Bad

Recipes for the caramel layer were all about the same: Dump one can of sweetened condensed milk, some butter, brown sugar, and corn syrup (my substitute for Britain’s golden syrup) into a saucepan and cook the mixture over medium heat while stirring until it turns thick and brown. Pour it over the baked shortbread, and that’s it.

Except that it wasn’t. My results were inconsistent. Sometimes the sauce broke; sometimes it didn’t­—and I couldn’t figure out why. I also noticed that when I added ½ teaspoon of salt to help offset the sweetness, breaking was pretty much a given. I was baffled.

It must have been fate that just at this point in my testing, Dr. Janice Johnson, a food scientist from Cargill who specializes in salt, paid the test kitchen a visit. I told her all about my breaking filling. She was not surprised. She explained that whey proteins in the sweetened condensed milk were one of the major classes of proteins responsible for keeping the caramel mixture emulsified but that those proteins had two enemies: salt and heat. As the mixture cooked, moisture evaporated, which increased the concentration of salt. At high concentrations of salt, proteins can fall out of solution and the caramel can take on a grainy appearance. At the same time, the heat was also damaging the proteins, which had already been compromised during the processing that transforms fresh milk into sweetened condensed milk.

If too much heat was a problem, maybe all my recipe needed was more precision. After several tests, I determined that I should cook the mixture to 236 degrees—hot enough to give it the proper texture but, most of the time, not so hot that it broke. But maddeningly, I found that I got different results when I used different brands of sweetened condensed milk, with some more prone to breaking than others.

Getting the caramel layer just right turned out to be trickier than we expected—until we reexamined the type of dairy we were using.

And then it occurred to me: If the compromised whey proteins in sweetened condensed milk were the problem, why not use fresh dairy since its proteins haven’t been damaged by processing? But subbing it in for the canned stuff would change the flavor profile of the filling, so I decided to try using it as a supplement. I knew from experience that milk didn’t have enough fat to prevent curdling at high temperatures, so it would have to be cream. Indeed, the whey in just ½ cup of cream proved to be enough to keep my caramel intact without compromising the flavor. Finally, I knew it would work to my satisfaction every time.

Recipe Testing: The Problem with Sweetened Condensed Milk

While developing the caramel filling for our Millionaire’s Shortbread, we found that fillings made with some brands of sweetened condensed milk were more prone to breaking—meaning that the butter separated out and pooled on the surface instead of staying emulsified—than others. When we took a closer look at the milks, we noticed that the ones that broke more often were darker in color than the ones that tended to stay emulsified. What was the connection? The temperature used in processing.

Some manufacturers process their milks at high temperatures, speeding up the Maillard reaction, which leads to more browned sugars and proteins, and damaging more whey proteins in the milk, which are key to keeping the fat emulsified. For a foolproof filling that won’t break, no matter the brand of sweetened condensed milk, we add fresh dairy. Just ½ cup of cream bolsters the mixture with enough undamaged whey proteins to prevent the butter from separating out.

From top to bottom, my version of this British classic was as perfect—and as gift-worthy—as I could have hoped.

Millionaire's Shortbread, Perfected

Most versions of millionaire’s shortbread rely on a labor-intensive shortbread layer, and they produce a caramel filling that separates during cooking and a chocolate layer that’s dull and soft. Here’s how we make a cookie that’s easier and a step above.

Chocolate with Shine and Snap

Grated chocolate and gentle heating in the microwave are the keys to an easy faux-tempered chocolate layer.

Filling that Doesn't Break

Supplementing the usual filling ingredients with a little heavy cream prevents the butter from separating out.

Quick, Pat-in-the-pan shortbread

Opting for melted butter means we need just a bowl and spoon, no food processor or stand mixer.

Keys to Success

  • Quick shortbread base

    Using melted butter rather than the usual solid form makes the shortbread base a quick dump-and-stir affair.
  • Chewy caramel filling

    Augmenting the traditional sweetened condensed milk with cream provides extra whey that prevents the filling from breaking.
  • Smooth, firm chocolate top

    Our faux tempering approach of melting the chocolate very carefully and adding grated chocolate at the end ensures that the top layer has a pleasing sheen and a snappy texture.