Make the Brownie Base:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a 9×9-inch baking pan with parchment paper for easy removal.
2. Mix the Wet Ingredients: In a large bowl, whisk together the melted butter and granulated sugar until smooth. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
3. Add Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, instant coffee granules, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until combined.
4. Bake the Brownies: Pour the brownie batter into the prepared pan, spreading it into an even layer. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out mostly clean (a few crumbs are okay). Let the brownies cool in the pan for about 10 minutes, then transfer them to a wire rack to cool completely.

Make the Mascarpone Layer:

5. Mix the Mascarpone Filling: In a medium bowl, whisk together the softened mascarpone cheese, powdered sugar, vanilla extract, and brewed espresso (and coffee liqueur, if using) until smooth and creamy.
6. Spread Over Brownies: Once the brownies have completely cooled, spread the mascarpone mixture evenly over the top of the brownies, smoothing it out with a spatula.

Finishing Touches:

7. Dust with Cocoa Powder: Use a sifter or fine-mesh strainer to dust the top of the brownies with unsweetened cocoa powder. This will give them the signature tiramisu look and flavor.
8. Optional Garnish: For extra flavor and decoration, sprinkle the brownies with chocolate shavings or cocoa nibs.

Chill and Serve:

9. Chill the Brownies: Refrigerate the brownies for at least 1 hour before serving to allow the mascarpone layer to set and firm up.
10. Serve: Slice the brownies into 12 squares and enjoy the decadent combination of chocolate and coffee flavors.