Step 1

Heat the oven to 350°F. Line the bottom and two opposites sides of an 8x8-inch square baking pan, preferably metal, with parchment paper so that it extends over the edges by a few inches. Lightly coat the pan and parchment with cooking spray or butter.

Step 2

Place 8 ounces finely chopped bittersweet chocolate and 1 stick cut-up unsalted butter in a medium saucepan. Cook over low heat, stirring frequently, until melted and smooth, 2 to 3 minutes. Remove the saucepan from the heat.

Step 3

Whisk in 2/3 cup granulated sugar and 1/2 cup packed dark brown sugar until combined. The mixture will be grainy. Whisk in 3 large eggs one at a time. Whisk in 2 teaspoons instant espresso powder, 1 teaspoon vanilla extract, and 1/2 teaspoon kosher salt until smooth.

Step 4

Stir in 1/2 cup all-purpose flour and 1/4 teaspoon baking powder until combined and no streaks of flour remain. Transfer to the baking pan and spread into an even layer.

Step 5

Bake until a tester inserted into the center comes out with just a few crumbs, 25 to 30 minutes. The edges should look firm and well-baked, and the center should be moist but not gooey. Let the brownies cool completely in the pan, about 3 hours.

Step 6

Beat 8 ounces room temperature mascarpone cheese and 1 cup cold heavy cream with a hand mixer in a medium bowl on medium speed until soft peaks form. Beat in 1/4 cup powdered sugar, 1 teaspoon vanilla extract, and 1 teaspoon instant espresso powder until slightly stiffer peaks form.

Step 7

Spread over the brownies in an even layer. Grasping the excess parchment, lift the brownie slab out of the pan and place on a cutting board. Place 1 tablespoon unsweetened cocoa powder in a fine mesh-strainer and dust over the top in an even layer. Cut into 12 to 16 pieces, wiping the knife clean between each cut for the neatest-looking slices.