Preheat oven to 350°F (175°C). Mix chilled butter, flour, sugar, and egg yolk to form dough. Roll out, cut into circles, press into tartlet pan, and bake for 15-20 minutes. Cool completely.

In a saucepan, heat heavy cream until simmering, pour over chopped dark chocolate, add brewed coffee. Stir until smooth and chill in the refrigerator for 30 minutes.

Combine room-temperature mascarpone, powdered sugar, and vanilla extract in a bowl. Whip with a hand mixer until light and fluffy.

Pipe coffee ganache into cooled tart shells, then pipe whipped mascarpone cream on top, creating a layered effect.

Refrigerate tartlets for at least 30 minutes before serving. Dust with cocoa powder just before serving.