SWEET TART DOUGH:

Sift together the confectioners' sugar, flour, and salt into a bowl. Place the butter in a food processor and process until smooth, about 15 seconds. Scatter the flour mixture over the butter, add the egg, and process just until the dough forms a mass; do not over mix. Turn the dough out onto the counter and divide it in 2. Shape each half into a disc, wrap in plastic wrap, and refrigerate for at least 2 hours or up to 1 month. Let the dough stand at room temperature for 30 minutes to soften. Lightly butter 2 (9 1/2-inch) fluted tart pans with removable bottoms. Dust a work surface lightly with flour. Dust one of the discs lightly with flour and, using a floured rolling pin, roll it out into a rough 12-inch circle. Lift the dough often, making sure that the work surface and dough are lightly floured at all times. Roll the dough up onto the rolling pin and gently unroll it over one of the prepared tart pans. Press the dough into the pan and roll the pin over the top of the pan to remove the excess dough. Prick the bottom of the tart shells all over with a fork. Chill the tart shells for 20 minutes. (The tart shells can be refrigerated for up to 24 hours.) To partially bake the tart shells, preheat the oven to 325 degrees. Lightly butter 2 pieces of aluminum foil large enough to generously line each tart pan. Line the tart shells with the foil, buttered side down, and fill with dried beans, rice or pie weights. Bake the tart shells for 15 minutes. Remove the foil and beans and continue baking for 5 minutes, until just set; the tart shells should have little or no color. Cool completely on a wire rack. To prebake the tart shells, preheat the oven to 325 degrees. Lightly butter 2 pieces of aluminum foil large enough to generously line each tart pan. Line the tart shells with the foil, buttered side down, and fill with dried beans, rice, or pie weights. Bake the tart shells for 15 minutes. Remove the foil and beans and continue baking for 8 to 10 minutes longer, until evenly golden brown. Cool completely on a wire rack.

CHOCOLATE TART:

Place the chocolate in a medium bowl and set a sieve over the bowl. Set aside. Put the cream in a medium saucepan. Scrape the seeds from the vanilla bean into the pan, add the bean, and bring to a boil. Pour the hot cream through the sieve onto the chopped chocolate. Allow to stand for 30 seconds to melt the chocolate, then whisk until the chocolate is completely melted and the mixture is smooth. Whisk in the egg yolks and butter until it is melted. Pour the filling into the baked tart shell. Refrigerated until set, at least 2 hours and up to 24 hours. Bring the tart to room temperature before serving.