To make the tart shell, whisk together the egg yolk, cream and vanilla in a small bowl. Set aside. Add the flour, cocoa powder, confectioners’ sugar and salt to the bowl of a food processor and pulse briefly to combine. Scatter the butter pieces over the top of the flour mixture and process until the mixture resembles coarse meal, about fifteen 1-second pulses. With the machine running, add the egg mixture and process just until the dough comes together. Turn the dough onto a piece of plastic wrap and press into a 6-inch disc. Wrap tightly and refrigerate for at least 1 hour or up to 48.

Remove the dough from the refrigerator. If it’s been longer than an hour, let it rest until it becomes workable. Unwrap the dough and place on a lightly floured surface. Roll into a 13-inch round. Press the dough into a 9-inch tart pan and remove the excess from the top. Place tart pan on a large plate and freeze for 30 minutes.

Adjust oven rack to middle position and heat oven to 375º. Set the dough-lined tart pan on a baking sheet and press a 12-inch square of aluminum foil into the frozen shell and over the edge and fill with pie weights. Bake for 30 minutes, rotating halfway through. Remove foil and weights and continue baking for another 5-8 minutes, just until the shell is set. Transfer to a wire rack to cool.

To make the filling, place the chocolate and butter in a large, heatproof bowl. Bring the cream to a simmer in a small saucepan and pour over the chocolate and butter. Let stand for 2-3 minutes to melt the chocolate. Whisk together until smooth, then whisk in the liqueur. Pour the filling into the prepared tart shell and smooth the top. Refrigerate, uncovered, until the filling is firm. To serve, transfer the tart to a cake stand or other serving platter. Use a thin, sharp knife to cut, wiping the blade between slices.