1) Preheat oven to 275°F and place rack in the center of the oven. Butter a 9-inch springflorm pan.
2) Place the chopped chocolate and butter in a large, microwave safe bowl and microwave on high in 30-second increments. Stir after each increment, continuing until the chocolate is completely melted. Allow to cool slightly, then whisk in the egg yolks.
3) Place the whites in a clean bowl and beat to soft peaks. Gradually add the sugar, beating until stiff and glossy.
4) Whisk ¼ of the whites into the chocolate mixture to lighten it.
5) Using a rubber spatula, gently fold the chocolate into the rest of the whites.
6) Pour it into the prepared pan and smooth the top.
7) Bake 40 to 50 minutes. The cake will have pulled away from the side of the pan and will be just set in the center. And allow to cool.
8) Dust the top of the cake with confectioners' sugar and serve.