Prepare the Cookie Dough:

1. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
2. Cream Butter and Sugar: In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy.
3. Add Wet Ingredients: Mix in the egg, vanilla extract, and espresso powder until fully combined.
4. Combine Everything: Gradually add the dry ingredients to the wet mixture, alternating with the milk, until a smooth dough forms.
5. Chill the Dough: Cover the dough with plastic wrap and refrigerate for at least 20 minutes.

Bake the Cookies:

6. Preheat the Oven: Set your oven to 350°F (175°C) and line a baking sheet with parchment paper.
7. Shape the Cookies: Scoop small portions of dough (about 1 tablespoon each) and roll into balls. Place them on the baking sheet, leaving space between each cookie.
8. Bake: Bake for 12-15 minutes, or until the edges are set but the centers remain soft.
9. Cool: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack.

Prepare the Mascarpone Filling:

10. Mix the Filling: In a small bowl, mix the mascarpone cheese, powdered sugar, and vanilla extract until smooth and creamy.
11. Chill if Needed: If the filling is too soft, refrigerate for 10 minutes to firm up.

Assemble the Tiramisu Cookies:

12. Fill the Cookies: Spread or pipe a small amount of mascarpone filling onto the flat side of half of the cookies. Top with the remaining cookies to create sandwich cookies.
13. Dust with Cocoa Powder: Lightly dust the tops with cocoa powder for the classic tiramisu look.
14. Garnish: Optionally, add chocolate shavings for an extra indulgent touch.