Make the Filling: Cream butter and powdered sugar. Mix in mascarpone, vanilla, and optional liqueur until smooth. Pipe 24 dollops (1 tsp each) onto parchment and freeze until firm (at least 20 min).

In a medium bowl, whisk together all dry dough ingredients (flour, cocoa, baking soda, salt, espresso powder, crushed instant coffee). Set aside.

Cream butter and sugar until light and fluffy. Mix in egg and vanilla.

Gradually add the dry ingredients to the wet, mixing only until combined. The dough will be thick.

Chill the dough for at_least_ 30 minutes in the refrigerator.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Place cocoa and powdered sugar in separate shallow bowls.

Form cookies: flatten 1.5 tbsp of dough, place a frozen mascarpone dollop in the center, and carefully seal the dough completely around it, checking for cracks.

Immediately roll the formed ball heavily first in cocoa powder, then heavily in powdered sugar. Space 2 inches apart on the baking sheets.

Bake for 9-11 minutes until crackled dramatically and edges are set, but the center still looks slightly soft.

Immediately dust the hot cookies with more cocoa powder using a sieve. Cool completely on the sheet for 15 minutes before moving. Serve completely cooled.