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

In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large bowl, cream together sugar and butter until light and fluffy.

Add the eggs, one at a time, beating well after each addition, and stir in the vanilla extract.

Mix in the sour cream and espresso until fully combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fill cupcake liners about 3/4 full with the batter. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Let the cupcakes cool completely on a wire rack.

For the coffee soak, combine espresso and coffee liqueur (if using) in a small bowl. Brush over the cooled cupcakes after piercing them with a fork.

For the frosting, beat mascarpone cheese, powdered sugar, and vanilla until smooth. In another bowl, whip the heavy cream to stiff peaks, then fold it into the mascarpone mixture.

Pipe or spread the frosting onto each cupcake.

Dust with cocoa powder and garnish with chocolate curls or grated chocolate.