Dark Mousse: In a bowl over barely simmering water, melt the bittersweet chocolate. Heat the milk in a saucepan until almost simmering and whisk it into the melted chocolate. Fold the whipped cream into the hot chocolate mixture; it will be rather soupy. Let cool slightly and transfer to a pastry bag or lockable plastic bag. Refrigerate at least 1 hour.

White Mousse: In a bowl over barely simmering water, melt the white chocolate. Heat the creme fraiche in a saucepan until almost simmering and whisk it into the melted chocolate. Fold the whipped cream into the hot chocolate mixture; it will be rather soupy. Let cool slightly and transfer to a pastry bag or lockable plastic bag. Refrigerate at least 1 hour. (The recipe can be made up to this point and kept refrigerated up to 2 days in advance.)

Insert the piece of plastic across the center (perpendicular to the bottom) of a ramekin or chocolate cup, so it divides the interior in half. If using plastic bags for piping, cut a corner off of each one. Pipe dark chocolate mousse on one side of the plastic, then white on the other. Pull out the plastic, and repeat. Refrigerate the ramekins until ready to serve.

When ready to serve, make chocolate curls by shaving milk chocolate into curls with a sturdy vegetable peeler, and don't worry if they break up into shards. Shards look nice, too. Remove the mousses from the refrigerator. If using collar molds, warm the metal with a hot, damp cloth and then lift off. Sprinkle the chocolate curls on top and serve.