1. Preheat the oven to 350°F and line two baking sheets with parchment paper.

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

3. In a large bowl, beat the softened butter, granulated sugar, and brown sugar until light and fluffy.

4. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract, espresso powder, and milk.

5. Add the dry ingredients to the wet ingredients and mix just until a soft dough forms.

6. Cover the dough and chill for 45 minutes.

7. Scoop the dough into 1 1/2 tablespoon portions and roll each one into a ball.

8. Coat each dough ball generously in powdered sugar.

9. Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart.

10. Bake for 10 to 12 minutes, until the tops are crackled and the edges are set.

11. Let the cookies cool completely on a wire rack.

12. In a clean bowl, beat the mascarpone cheese and heavy cream until smooth and fluffy.

13. Spoon or pipe the filling onto half of the cookies, then top with the remaining cookies if making sandwiches.

14. Dust lightly with cocoa powder before serving.