“The lime flavor was very strong, and the crust adds a nice little crunch.” —Suriya, recipe tester, age 10
For the crust: Adjust oven rack to middle position and heat oven to 325 degrees. Make aluminum foil sling for 8-inch square metal baking pan. Spray foil with vegetable oil spray.
Add cracker pieces, flour, sugar, and ⅛ teaspoon salt to food processor and lock lid into place. Hold down pulse button for 1 second, then release. Repeat until crackers are broken into small pieces, about five 1-second pulses.
Turn on processor and process until crackers are finely ground, about 30 seconds. Stop processor. Remove lid, add melted butter to processor, and lock lid back into place. Pulse until butter is combined with crumbs, about ten 1-second pulses.
Remove lid and carefully remove processor blade (ask an adult for help). Use rubber spatula to scrape crumb mixture into foil-lined baking pan.
Use your hands to press crumbs into even layer covering bottom of baking pan, then use bottom of dry measuring cup to press crumbs firmly into pan until very flat.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let crust cool for at least 15 minutes. (Do not turn off oven.)
For the filling: Meanwhile, in large bowl, whisk condensed milk, softened cream cheese, lime zest and juice, egg yolk, and pinch salt until well combined and smooth.
Pour filling evenly over cooled crust. Use clean rubber spatula to scrape all filling from bowl. Spread filling into even layer to corners of pan and smooth top.
Place baking pan in oven and bake until filling is set and no longer jiggles when pan is shaken gently (ask an adult for help), 15 to 20 minutes.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let bars cool in pan for 2 hours. Cover baking pan with plastic wrap and refrigerate until bars are chilled and firm, at least 2 hours or up to 2 days.
Remove bars from refrigerator and remove plastic. Use foil to lift bars out of baking pan and transfer to cutting board. Cut into squares. Top squares with toasted coconut or Whipped Cream (if using) and serve.
Toasted coconut is a great addition to baked goods such as cookies or bars or as a topping for ice cream or oatmeal. It can sometimes burn when you toast it in the oven, so we like to use the microwave, where you can keep a close eye on it as it cooks.
To make toasted coconut: Spread ½ cup sweetened shredded coconut into even layer on large plate. Cook in microwave until golden brown, 3 to 5 minutes, stopping to stir with rubber spatula every 30 seconds. Use oven mitts to remove plate from microwave (careful—plate will be HOT!) and let cool to room temperature.
Key limes are smaller, yellower cousins to the bigger, darker green Persian limes you usually see in the grocery store. Key limes are more tart, with fragrant, floral juice. They used to be grown widely in the Florida Keys, and cooks who lived there invented key lime pie as a way to use the juice. But to make these Key Lime Bars, you don’t need key limes—they can be hard to find and are so small that you’d have to squeeze 20 of them to get ½ cup of juice! Regular Persian limes work just fine, but whatever you do, don’t use bottled lime juice in this recipe (fresh juice has much better flavor!).