Step 1

Adjust oven rack to middle position and preheat to 350°F. Line a 9-by-13-by-2-inch aluminum baking pan with two crisscrossed sheets of foil so that the bottom and long sides are covered, leaving an overhang on the long sides, and grease lightly. Sift flour and cocoa together (if using cup measures, spoon into the cups and level with a knife before sifting).

Step 2

In a 2-quart stainless steel saucier, melt butter over medium-low heat. Increase to medium and simmer, stirring with a heat-resistant spatula while the butter hisses and pops. Continue cooking and stirring, scraping up any brown bits that form in the bottom of the pan, until the butter is golden yellow and perfectly silent. Remove from heat and stir in chocolate; the mixture will seem quite thin.

Step 3

Combine white sugar, brown sugar, salt, eggs, vanilla, and instant espresso, if using, in the bowl of a stand mixer fitted with the whisk attachment. Whip on medium-high speed until thick and fluffy, about 8 minutes. Reduce to low and pour in warm chocolate-butter. Once incorporated, add flour/cocoa and continue until well combined. Fold with a flexible spatula, to ensure the batter is well mixed from the bottom up.

Step 4

Pour into the prepared pan and bake until the brownies are glossy and just barely firm (like the soft part of your forearm), about 25 minutes (or 205°F). Cool to room temperature.

Step 5

Tug the foil loose and lift the brownies from the pan. Cut into twenty-four 21/2-inch squares. Store in an airtight container with a sheet of wax paper between each layer for up to 1 week at room temperature. Leftover brownies can be chopped into bite-sized pieces and frozen for up to 6 months, a fantastic mix-in for your favorite homemade ice cream.