In a mixing bowl, combine the flour, cocoa powder, powdered sugar, and salt. Add the chilled, cubed butter and mix until only small clumps remain.

Incorporate the egg yolk, mixing until the dough starts to come together.

Wrap the dough in plastic wrap and refrigerate for 30 minutes.

While the dough is chilling, preheat your oven to 350°F (175°C).

Once chilled, roll out the dough on a floured surface to about 1/8-inch thickness.

Use a round cutter or glass to cut out circles and place them into a mini tart pan.

Prick the bottoms of the tart shells with a fork and bake for 10-12 minutes, or until the edges are firm.

Allow to cool completely.

In a small saucepan over low heat, warm the heavy cream until just simmering.

Remove from heat, add the chopped chocolate and vanilla extract, and stir until smooth and glossy.

Pour the chocolate ganache into the cooled tart shells.

Allow them to set at room temperature or refrigerate for about 1 hour until firm.

Once set, top your Mini Chocolate Tarts with fresh fruits, whipped cream, or your choice of toppings for an extra flavor kick.