Prepare the Dough: Beat together butter and granulated sugar until fluffy. Add eggs and vanilla, then sift in flour, espresso powder, and salt, mixing until a thick dough forms.

Chill the Dough: Wrap dough in plastic wrap and refrigerate for 30 minutes. Preheat your oven to 175°C (350°F).

Shape the Cookies: Scoop dough into tablespoon-sized portions, roll into balls, and coat in powdered sugar.

Bake the Cookies: Place on a baking sheet and bake for 10-12 minutes until puffed with cracks.

Create Wells for Filling: Use a teaspoon to press a well into each cookie while warm.

Cool Completely: Allow cookies to cool on a wire rack for about 15-20 minutes.

Prepare the Mascarpone Filling: Combine mascarpone cheese and heavy cream until thick, adding espresso powder if desired.

Fill the Cookies: Pipe or spoon the filling into the wells of cooled cookies.

Chill Before Serving: Refrigerate filled cookies for about 15 minutes before serving.