Preheat oven to 325 degrees. Grease an 8 x 2 inch round pan (not a springform) and line the bottom with wax or parchment paper.
1 pound bittersweet or semisweet chocolate, coarsely chopped
10 tablespoons unsalted butter, cut into 10 pieces
5 large egg yolks
5 large egg whites
1/4 tsp. cream of tartar
1 tablespoon sugar