✓ Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.

✓ In a large bowl, combine the flour, cocoa powder, powdered sugar, and salt. Mix well until thoroughly blended.

✓ Add the chilled butter cubes and use your fingers or a pastry blender to work them into the dry ingredients until you achieve a coarse meal-like consistency.

✓ Add the egg yolk to the bowl and mix gently until the dough comes together in a ball. If it’s too sticky, add a tablespoon of cold water; if too crumbly, add another tablespoon of flour.

✓ On a lightly floured surface, roll out the dough into a circle about 1/8 inch thick. Carefully transfer it to a tart pan with a removable bottom and trim any excess dough around the edges.

✓ Prick the base of the crust several times with a fork to prevent it from puffing up too much during baking.

✓ Bake the crust for 15-20 minutes or until golden brown, then remove from the oven and let cool on a wire rack.

✓ In a separate bowl, mix the mascarpone cheese, heavy cream, cooled espresso, vanilla extract, and coffee liqueur (if using) until smooth.

✓ Spread the filling evenly over the cooled crust, leaving about 1/4 inch around the edges.

✓ Place the tart on the middle rack of your preheated oven and bake for an additional 25-30 minutes or until the filling is set but still slightly jiggly in the center.

✓ Once cooled to room temperature, dust the top with cocoa powder and garnish with chocolate shavings and coffee beans if desired. Let the tart rest in the refrigerator for at least 30 minutes before serving.