Preheat the oven to 350 degrees F. Brush 4 to 6 individual souffle dishes (or a 7-cup souffle dish) well with melted butter. Put the sugar inside one dish and toss to coat, then tip the excess sugar into the next dish and repeat. Place the dishes in the refrigerator to chill.

Reserving 1/3 cup of the milk, place the remaining 1 cup in a large saucepan. Add the vanilla bean and heat just to a boil, then turn off the heat and set aside to let the vanilla bean infuse for 10 minutes. Meanwhile, in a medium bowl, whisk the 1/3 cup granulated sugar, the flour, and the reserved 1/3 cup milk. Add a little of the hot milk and whisk to combine, then add the flour mixture to the hot milk mixture in the saucepan. Bring to a simmer over medium heat, whisking occasionally. Simmer just until thickened, about 1 minute. Turn off the heat. Add the butter, stir, cover, and set aside 15 minutes. Remove the vanilla bean and whisk in the egg yolks.

In a mixer fitted with a whisk attachment (or using a hand mixer), whip the egg whites in a clean dry bowl until soft peaks form. Add the remaining 1 1/2 tablespoons sugar and continue whipping until stiff and glossy. Fold into the milk mixture.

Pour the mixture into the prepared dishes, filling them 3/4 full. Run your thumb around the inside rim of each dish, making a shallow "moat" around the edge of the batter. This will help the souffles to rise straight up. Dust the surfaces with confectioners' sugar. (The souffles can stand at this point for up to 30 minutes.) Bake in the center of the oven until puffed and light golden brown, 20 to 30 minutes. Do not open the oven door unless absolutely necessary.

Satin Chocolate Sauce: Combine the 2 chocolates over simmering water in the top half of a double boiler. Stir constantly until melted, then whisk in the syrup and water without removing the double boiler from the heat. Whisk until smooth and shiny. (The sauce can be made up to 48 hours in advance and refrigerated. To rewarm, stir over low heat or heat in a microwave.) At the table, serve the hot souffles by poking a hole in each one and pouring chocolate sauce into the hole, then drizzling sauce over the tops. For a large souffle, spoon onto dessert plates and drizzle sauce on each serving.