In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt. In a separate bowl, cream the unsalted butter until smooth, then mix in the eggs and espresso powder until well incorporated. Combine the wet and dry ingredients, mixing until a soft dough forms. Cover the dough with plastic wrap and chill in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Remove the chilled dough from the refrigerator and roll it into 1-inch balls. Roll each ball generously in powdered sugar and place on the prepared baking sheet, leaving space for spreading.

Bake the cookies for 10-12 minutes, until the edges are golden while centers remain soft. Press a finger into each cookie to create an indentation.

While the cookies cool, whip together mascarpone cheese and heavy cream until glossy and thick. Don't over-whip. Set aside in the refrigerator.

After cooling, fill the indented cookies with the mascarpone filling and optionally dust with cocoa powder before serving.