Preheat this oven to 350°F.

In a food processor, combine the flour, dark cocoa powder, and granulated sugar. Pulse a few times to mix.

Add the cold butter and pulse until the mixture resembles fine crumbs.

Add the egg and pulse until the dough just comes together. If needed, add 1 tablespoon of cold water to help bind the dough.

Divide the mixture evenly among 6 small tartlet pans. Press the crumbs firmly into the bottom and sides of each pan to form a crust. Trim any excess around the edges.

Prick the bottoms of the tart shells with a fork then fill with parchment paper and pie weights to prevent puffing. Bake for 12-15 minutes, or until set. Allow to cool completely.

Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer (do not boil).

Remove from heat and pour over the chopped semi-sweet chocolate in a heatproof bowl. Let sit for 2-3 minutes, then whisk until smooth and glossy.

Pour the chocolate filling into the cooled tart shells, filling them just below the rim. Smooth the tops with a spatula if necessary.

Refrigerate the tartlets for at least 1 hour, or until the filling is set. Serve chilled, garnished with leftover chocolate drizzle, whipped cream, berries, caramel sauce, sprinkles, melted chocolate or a pinch of fluer de sel, if desired.