Sift flour and cocoa together.
Melt the butter over the stovetop in a medium heavy-bottomed pan over medium-low heat. Once it is melted, increase the heat to medium and allow to simmer, stirring frequently with a spatula. Cook until it stops hissing, scraping up any browned bits from the bottom of the pan. Remove from the heat and stir in the chocolate until fully melted and combined
Combine the sugars, salt, eggs, vanilla, and instant espresso powder into the bowl of a stand mixer fitted with the whisk attachment. Whip together on medium-high speed for 8 minutes, or until very thick and fluffy. Turn speed down to low and slowly pour in the chocolate-butter mixture. Once that is incorporated, add in the flour/cocoa mixture and mix until well combined. Fold the batter together with a rubber spatula to ensure that it is well mixed and that there are no pockets of flour hiding in it.
Pour the batter into the prepared baking pan. Bake for 25 minutes, or until the brownies are glossy and just barely firm. Allow to cool completely in the pan.
