• Preheat your oven to 350°F (175°C).
• Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the sides for easy lifting later, and lightly spray the parchment. Set the pan aside.

Make the brownie layer:

• Melt 1 cup unsalted butter with 3 ounces dark or semisweet chocolate; see Notes below for instructions. Stir to combine until the chocolate is fully melted.
• In a large bowl, whisk together the butter and chocolate mixture, 1 cup white sugar and 1 cup packed brown sugar until smooth and the sugars mostly dissolve.
• Add 4 large eggs and 1 tablespoon vanilla extract, and whisk again until fully combined and glossy.
• Sift in 1 cup all-purpose flour, ½ cup unsweetened cocoa powder, 1 teaspoon espresso powder, and ½ teaspoon salt, and stir until well mixed, but don’t overdo it. You can sift them before in a separate bowl or have the ingredients measured and sift them directly over the butter mixture when prompted (my choice). 
• Pour the brownie batter into the prepared pan and spread it into an even layer.
• Bake on the middle rack for 15-18 minutes, or until the brownies are almost set in the center but a tester or toothpick comes out with moist crumbs attached. Avoid overbaking; they’ll continue to firm up as they cool.
• Let the brownies cool completely in the pan. Then, cover and refrigerate the pan until cold. For me, it's 30 minutes to 1 hour, but it depends on the temperature of your fridge.

In the meantime, make the mascarpone filling:

• In a medium bowl, beat or whip 1 cup heavy cream until medium peaks (the mixture is thick but the tip falls when the beaters are lifted) with an electric mixer. Set it aside.
• In a large bowl, beat 18 ounces mascarpone cheese, ¼ cup white sugar, 2 teaspoons vanilla extract, and 1 tablespoon espresso powder until smooth.
• Add the whipped cream and fold it gently using a spatula; integrate completely. No streaks of cream should remain.

Assemble the brownies:

• In a small bowl, mix ½ cup espresso coffee with 2 tablespoons coffee liqueur and soak the entire cold brownie surface with it. Use a pastry brush, don’t pour it, or it will be uneven.
• Spread the mascarpone mixture over the brownie and smooth to have an even layer.
• Refrigerate it, covered in plastic wrap, for at least 4 hours or overnight.
• When ready to serve, remove carefully from the pan with the help of the paper and place on a flat surface.
• Dust the top with 2 tablespoons unsweetened cocoa powder. slice and serve.