Preheat your oven to 350°F (175°C). In a mixing bowl, combine butter, flour, sugar, and egg yolk until a dough forms. Press the dough into tartlet molds evenly and prick the bottoms with a fork to prevent bubbling. Bake for 12-15 minutes, or until golden brown. Remove from the oven and allow them to cool completely on a wire rack.

In a saucepan, heat heavy cream over medium heat until it starts to simmer, then pour it over finely chopped dark chocolate in a bowl. Add fresh brewed coffee. Wait a minute before stirring until smooth and glossy. Allow the ganache to cool at room temperature for about 30 minutes, then refrigerate to chill until thickened.

In a separate bowl, whisk together mascarpone cheese, powdered sugar, and vanilla extract until smooth and creamy. Once ganache has thickened, pipe or spoon mascarpone cream over ganache in cooled tart shells.

Dust the top of the mascarpone layer with cocoa powder and add chocolate shavings. Chill tartlets in the refrigerator for at least 1 hour before serving.