In a food processor, pulse together the flour, cocoa powder, powdered sugar, and salt until combined.
Add the cold, cubed butter and pulse until pea-sized crumbs form. (Alternatively, cut the butter in by hand with cold hands).
Add the egg and vanilla extract, pulsing just until a dough forms. Avoid overmixing.
Shape the dough into a disk, wrap in plastic wrap, and chill in the fridge for at least 1 hour.
Roll the chilled dough out on a floured surface. Cut small rounds using a round cutter or bowl that is about 1 inch larger than your mini tart pans.
Press the pastry gently into each mini tart pan, trim the edges, and dock the bottoms with a fork. Freeze the shaped crusts for at least 20 minutes.
Bake in a preheated oven at 180°C (350°F) for 15–20 minutes. Remove and let cool completely.
Make the chocolate ganache
Combine the chopped dark chocolate and heavy cream in a small bowl. Melt in the microwave using 30-second pulses or set over a double boiler (bain-marie).
Let the mixture sit undisturbed for 10 minutes, then whisk until smooth, glossy, and silky.
Pour the ganache into the cooled tart shells until almost full. Transfer to the refrigerator for 2–3 hours to set completely.
Notes
Butter Temperature: Ensure the butter is strictly ice-cold before mixing to achieve the signature crisp, tender shortbread texture.
Chocolate Quality: Use high-quality chopped chocolate bars (at least 70% cocoa) instead of chocolate chips. Chips contain stabilizers that disrupt a smooth ganache emulsion.
Crust Docking: Always dock the unbaked pastry bottoms with a fork to release steam and prevent the shells from warping or bubbling during baking.
Make-Ahead Flexibility: Unbaked shaped crusts can be frozen for up to 1 month and baked straight from the freezer by adding 2 minutes to the bake time.
Storage & Freezing: Store assembled tarts in an airtight container in the refrigerator for up to 4 days. For longer storage, freeze in an airtight container for up to 3 months and thaw overnight in the fridge before serving.