Make the Chocolate Tart Shell

Preheat the oven to 400°F (200°C).

Whisk together flour, cocoa powder, sugar, and salt in a large bowl.

Rub in butter until the mixture resembles a coarse meal.

Stir in the beaten egg and quickly knead the dough, just until it comes together.

Wrap and refrigerate for 30 minutes to let the dough firm up a bit.

Once chilled, on a lightly floured surface, roll out the dough and line a 9-inch (23cm) tart pan with it and prick all over with a fork. Refrigerate for 15 minutes.

Line the dough with parchment and fill with pie weights.

Bake for 15 minutes, then remove the weights and parchment and continue to bake for another 10 minutes, or until fully baked.

Make the Chocolate Filling: in a saucepan over medium heat, melt the chocolate and butter together. Let cool for five minutes.

Whisk the sugar, egg, and yolks together in a bowl and then gradually whisk the egg mixture into the chocolate mixture until smooth.

Once the crust is baked, remove it from the oven, pour the filling into the hot crust and return to the oven. Bake for five minutes.

Let cool and set, loosely covered, at room temperature for 4 hours (or overnight).

Dust with cocoa powder before serving.

Store any leftovers in an airtight container in the refrigerator for up to 3 days. This tart can also be frozen in an airtight container for up to 2 months. Bring up to room temperature for an hour or two before serving.