Ingredients

1. 1/2 pound bittersweet chocolate, finely chopped
2. 1 stick (8 tablespoons) unsalted butter, cut into small pieces
3. 4 large eggs, separated
4. 3/4 cup plus 2 tablespoons granulated sugar
5. 1 teaspoon confectioners' sugar
6. Unsweetened whipped cream, for serving (optional)

Directions

1. Preheat the oven to 375°. Butter and flour a baking sheet. Butter the insides of 6 metal rings that measure 3 inches wide and 2 inches high (see Note). Dip the rings in flour and set them on the prepared baking sheet.
2. In a medium bowl set over a saucepan of barely simmering water, melt the chocolate and butter, stirring occasionally, until smooth, about 5 minutes. Let cool slightly. 
3. In another bowl, lightly beat the egg yolks. Using a rubber spatula, stir in the chocolate until blended. 
4. In a large bowl, using an electric mixer, beat the egg whites until soft peaks form. Gradually beat in the granulated sugar until the whites are firm and glossy. Gently fold the chocolate mixture into the whites just until no white streaks remain. Cover and refrigerate for 4 hours. 
5. Spoon the mixture into the prepared rings and bake for about 18 minutes until the tops are firm and a knife inserted between the inside of a ring and the cake comes out clean. Using a wide metal spatula, transfer the tortes to individual plates and remove the rings. The tortes will sink slightly and crack. Sift the confectioners' sugar over the tops and serve at once with whipped cream. 
6.  

Notes

Metal cake rings are available at kitchen supply stores. They must be at least 2 inches high. Alternatively, 8-ounce pineapple cans with the tops and bottoms removed or any other similarly sized cans can be used.