Preheat your oven to 375°F (190°C) and line two baking sheets with parchment paper.

Cream together the softened unsalted butter and granulated sugar until light and fluffy, about 3-5 minutes. Beat in the egg, vanilla, and rum extracts until smooth.

Whisk together the flour, baking powder, and salt in a separate bowl. Gradually add the dry mixture to the creamed mixture, mixing until just combined. Chill the dough for at least 30 minutes.

Scoop tablespoons of dough, roll into balls, and roll in extra granulated sugar. Flatten them into disks about ½-inch thick.

Bake for 9-10 minutes until the edges are set but centers are slightly soft. Cool on the baking sheets for a few minutes before transferring to a wire rack.

Prepare the filling by blending mascarpone and softened butter until smooth. Add the dissolved espresso and gradually incorporate powdered sugar until fluffy.

Once cookies are cool, pipe or spread the mascarpone filling on half of the cookies and sandwich with the other half.

Dust the tops with unsweetened cocoa powder before serving.