In a food processor, combine the sugar and butter. Process until creamy.

Add vanilla, salt, and cocoa. Process until the mixture forms a smooth paste.

Add the flour, and process until just blended.

Turn out the dough. Place it on a sheet of plastic wrap or waxed paper. Shape into an 8-inch disk, wrap, and chill in the refrigerator for an hour.

Roll out the crust into an 11-inch circle, and fit into a 9 1/2 inch tart pan, making strong sides. Chill at least 30 minutes.

Preheat oven to 350°F.

Prick the bottom of the crust with a fork, then pop it in the oven for 12-14 minutes, or until the pastry is dry to the touch. Set on a rack to cool.

Place the chocolate in a heatproof bowl.

On medium heat, heat the cream in a sauce pan until bubbles form along the edge of the pan.

Pour cream over the chocolate. Let stand for 30 seconds. Stir until melted and smooth.

Pour the chocolate into the tart shell. Smooth it out on top so it looks nice, and refrigerate until it is set, about 3 -5 hours.