Getting Ready: Adjust oven rack to the middle position and preheat to 350°F. Line a 9x13x2 aluminum baking pan with foil to cover the bottom and long sides of the pan. If you like, grease lightly with pan spray.
In a 3-quart stainless steel sauce pan, melt the butter at medium heat. Cook and stir, scraping up any brown bits that form along the pan, until the butter is golden yellow. Remove from heat and stir in the chocolate; it will seem thin.
Making the Batter: 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 until extremely thick and fluffy, about 8 minutes.
Meanwhile, sift together the flour and cocoa powder. When the foamed eggs are fluffy and thick, reduce speed to low and pour in the warm chocolate-butter. Once incorporated, add the cocoa-flour all at once and continue mixing until roughly combined. Finish with a flexible spatula, scraping and folding to ensure the batter is well mixed from the bottom up.
Pour into the prepared pan and bake until the brownies are glossy and just barely firm, about 30 minutes or to an internal temperature of 205°F. The timing of this recipe is based on anodized aluminum, and will vary significantly with glass, ceramic, or non-stick pans, materials that will also affect the brownie's consistency.
Cool the brownies to room temperature to allow their crumb to set before slicing. Slide a butter knife between the brownies and the short ends of the pan; gently tug on the foil, then lift to remove the brownies from the pan. Cut into twenty-four 2 1/2-inch squares, and store in an airtight container with a sheet of wax paper between each layer, about 1 week at room temperature.
