01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
03 - In a large bowl, cream the butter and sugar together until light and fluffy, about 2 minutes. Beat in the egg and vanilla extract.
04 - Stir in the dissolved espresso and milk until well combined.
05 - Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated.
06 - Scoop tablespoon-sized portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.
07 - Bake for 10-12 minutes, until the edges are set but the centers are still soft. Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
08 - In a medium bowl, beat mascarpone, cream cheese, and butter until smooth. Add powdered sugar and vanilla extract; beat until creamy and fluffy.
09 - Once cookies are fully cooled, spread a generous layer of mascarpone frosting on each.
10 - Lightly dust frosted cookies with unsweetened cocoa powder just before serving.