1¼ cups all-purpose flour
1 teaspoon salt
2 tablespoons dark unsweetened cocoa powder
11 ounces dark chocolate (60 to 72%), coarsely chopped
1 cup (2 sticks) unsalted butter, cut into 1-inch pieces
1 teaspoon instant espresso powder
1½ cups granulated sugar
½ cup firmly packed light brown sugar
5 eggs, at room temperature
2 teaspoons pure vanilla extract
Heat oven to 350º and butter the sides and bottom of a 9-by-13-inch glass baking pan. Whisk together the flour, salt and cocoa powder in a medium bowl and set aside.
Place the chocolate, butter and espresso powder in a large bowl and set over a pot of simmering water. Cook, stirring occasionally, until the mixture is completely melted and smooth. Turn off heat, but keep the bowl over the water. Whisk in the sugars until completely combined, then remove bowl from the pot. If the mixture isn’t room temperature at this point, allow it to cool slightly until it reaches room temperature.
Whisk 3 eggs into the chocolate mixture, taking care not to overbeat them. Whisk in the remaining 2 eggs, then stir in the vanilla. Using a rubber spatula, gradually fold in the flour until just incorporated.
Pour batter into the prepared pan and smooth the top. Bake in the center of the oven for 30 minutes, rotating halfway through, until a toothpick comes out with just a few moist crumbs attached. Allow to cool, then cut into squares and serve. Brownies can be kept, tightly wrapped, at room temperature for up to 3 days (like they’ll last that long).