The difference between chocolate mousse and the filling of this crémeux (trans. creamy) tart is that the latter doesn't contain whipped egg whites or whipped cream, and so has a denser, less airy consistency. Which makes it an excellent filling for a tart shell.

The crust here is a French pâte sucrée from scratch, but you could simplify the recipe significantly by purchasing a pre-made shell.

Note that there are two teaspoons of sugar in the crust, and none added to the filling. All the sweetness comes from the semi-sweet chocolate.

Ingredients

TART SHELL
1-1/2 cups all-purpose flour
8 tablespoons (one stick) unsalted butter, cut into small cubes and chilled
2 teaspoons sugar
1/4 teaspoon salt
3-4 tablespoons of ice water

FILLING
1 cup half-and-half
4 large egg yolks
1 tablespoon unsweetened Dutch Process cocoa powder
1/4 teaspoon salt
12 ounces semi-sweet chocolate, chopped fine
1 teaspoon vanilla extract
10 tablespoons unsalted butter, melted and hot

MAKE TART SHELL
Add the flour, chilled butter, salt, and sugar to the bowl of a food processor. Pulse repeatedly until the mixture has the texture of coarse crumbs. Add the ice water and pulse until the dough just starts to stick together.

Turn the dough onto a floured surface. Form the dough into a flat disc, wrap in parchment or wax paper, and refrigerate for an hour.

Make and bake the pie shell. Roll out the disc of dough to a 12-inch circle. Transfer to a tart pan, using fingers to gently push the crust into the pan's corner. Trim and crimp the edge, and prick the bottom of the crust bottom a dozen times. Chill for 20-30 minutes, or up to 24 hours.

When you're ready to bake, preheat the oven to 375F. Line the crust with parchment paper, then fill with pie weights (or beans or rice). Bake for about 20 minutes. Remove weights and parchment, return to oven and bake for about 20 more minutes, watching carefully as pastry can brown quickly. Set aside to cool.

MAKE FILLING
In a small sauce pan, whisk together the half-and-half, egg yolks, cocoa powder, and salt.

Heat the pan over medium-low heat, stirring & scraping the bottom constantly with rubber spatula, for about 5-7 minutes, until the mixture has noticeably thickened and coats the spatula. If you have a thermometer, heat until the temperature is in the 170-175F range.

Remove from heat. Stir in the chocolate pieces and the vanilla until the mixture is smooth. Add the melted butter and stir to combine thoroughly.

ASSEMBLE
Pour the chocolate mixture into your cooled tart shell, cool to room temperature then refrigerate for at least two hours and up to a whole day.

Serve with whipped cream if you like.