The first decision I faced was the mixing method. Blondie recipes typically call either for creaming butter and sugar and then adding the eggs and dry ingredients or for mixing melted butter with the sugar and eggs before incorporating the dry ingredients. Since creaming the butter and sugar incorporates air that results in a cakey texture, I settled on the second method, which leads to a denser, chewier bar.
Next order of business: boosting butterscotch flavor. Most blondie recipes call for light brown sugar, and I wondered if using dark brown sugar instead would help just a little. Working with a standard recipe from my research—a 1:1 ratio of sugar to flour, salt, baking powder, a couple of eggs, vanilla extract, and one stick of melted butter—I ran a quick side-by-side test. Interestingly, most tasters panned the dark brown sugar for making the blondies taste more like molasses than butterscotch; they also preferred the more golden color of the bars made with light brown sugar.
But I did have something more promising in mind to try: Instead of just melting the butter, I would brown it to create warm, toasty, nutty flavors. So for my next batch of bars, I cooked the butter in a skillet until the milk solids had turned a dark golden brown and had a nutty fragrance, combined it with the other ingredients, and then baked the batter in a 13 by 9-inch pan in a 350-degree oven until lightly golden. This was a big step in the right direction: The blondies tasted decidedly more nutty and rich.
A colleague had another idea: Since vanilla extract's earthy, woodsy notes mirror and complement butterscotch flavors, why not try more of it? Doubling the amount of vanilla worked so well to deepen the warm caramel flavor in the bars that I kept going and tripled it.
Increasing the vanilla gave me the idea to increase another ingredient: the salt. Salt is included in most sweet applications because it helps sharpen all the flavors, particularly nutty, buttery ones. When I doubled it from ½ teaspoon to 1 teaspoon, those flavors stood out a little more in my blondies and the sweetness a little less.