Grease and flour an 8 inch springform pan or round cake pan.
Preheat oven to 300 degrees F. (Yes, you read that correctly. This cake cooks at a low temp.)
Melt chocolate in the top of a double boiler or in a metal bowl over water that is just boiling.
Beat the eggs yolks, half of the powdered sugar, the granulated sugar and vanilla. Then, beat in the melted butter and chocolate.
Sift in the flour and cornstarch and mix well.
In a separate bowl, beat the egg whites until stiff and then whisk in the other half of the powdered sugar. Using a thin spatula, gently fold the egg white mixture into the chocolate mixture until a uniform color is achieved.
Pour the batter into the prepared springform or cake pan and bake at 300 degrees F for 40 minutes. Wait until the cake cools completely before removing it from the pan. Turn the cake over onto a serving platter and sprinkle generously with powdered sugar.