Preheat the oven to 350°F (175°C). Line a standard muffin tin with cupcake liners.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Begin and end with the flour mixture. Mix until just combined.

Fill each cupcake liner about 2/3 full with the batter. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

In a separate bowl, combine the cooled brewed coffee and coffee liqueur (if using). Once the cupcakes are cool, poke holes in the tops of each cupcake with a toothpick and drizzle the coffee mixture over them, allowing it to soak in.

In another bowl, beat the mascarpone cheese, heavy whipping cream, and powdered sugar together until stiff peaks form. Frost the cooled cupcakes generously with the mascarpone mixture.

Dust the tops of the frosted cupcakes with cocoa powder before serving.