Preheat the oven to 350°F (175°C). Mix flour, sugar, and salt in a bowl. Cut in butter, add egg yolk, knead until dough forms a ball. Roll out, cut circles, and place in molds. Bake for 12-15 minutes until golden, then cool.

For the coffee ganache, heat cream until simmering. Remove from heat, stir in chocolate until smooth, add brewed coffee, mix well, let cool for 30 minutes, then refrigerate to set.

Blend mascarpone cheese, powdered sugar, and vanilla until smooth and thick (2-3 minutes). Refrigerate while ganache sets.

Assemble by filling tart shells with ganache. Pipe mascarpone cream on top, creating swirls for presentation.

Chill assembled tartlets for at least 30 minutes. Dust with cocoa powder before serving. Enjoy chilled.