Preheat the oven to 350°F (175°C).
In a medium bowl, mix together flour, baking powder, and salt.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the egg and the cooled coffee until well combined.
Gradually add the dry mixture to the wet ingredients, mixing until just combined.
Scoop the dough into muffin tins lined with cupcake liners, filling each about 2/3 full.
Bake for 12–15 minutes or until lightly golden around the edges.
Allow the cookie cups to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
In a small bowl, mix mascarpone cheese, powdered sugar, and vanilla extract until smooth and creamy.
Once the cookie cups are cool, pipe or spoon the mascarpone filling into each cup.
Dust with cocoa powder before serving.