14 squares (14 oz) semisweet chocolate, chopped
2 squares (2 oz) unsweetened chocolate, chopped
1 cup butter (2 sticks; do not use margarine)
9 large eggs, separated
12 cup granulated sugar
1/4 tsp. cream of tarter
Preheat oven to 300 degrees. Remove bottom of 9 by 3 spring form pan; cover with foil, wrapping foil around back. Replace pan bottom. Grease and flour foil bottom and side of pan.
In heavy 2-quart saucepan, melt semisweet and unsweetened chocolates and butter over low heat, stirring frequently, until smooth. Pour chocolate mixture into large bowl.
In small bowl, with mixer at high speed, beat egg yolks and granulated sugar until very thick and lemon-colored, about 10 minutes. With rubber spatula, stir egg-yolk mixture into chocolate mixture until blended. Wash and dry beaters.
In separate large bowl, with clean beaters and with mixer at high speed, beat egg whites and cream of tarter until soft peaks form when beaters are lifted. With rubber spatula, gently fold beaten egg whites, one-third at a time, into chocolate mixture just until blended.
Scrape batter into prepared pan; spread evenly. Bake 35 minutes (Do not over bake; cake will firm upon standing and chilling.) Cool completely in pan on wire rack; refrigerate overnight in pan.
Run thin knife, rinsed under very hot water and dried, around cake to loosen from side of pan; remove side of pan. Invert onto cake plate; unwrap foil from pan bottom and lift off pan. Carefully peel foil away from cake.
To serve, let cake stand at room temperature 1 hour. Dust with confectioners' sugar. Or dust heavily with confectioners' sugar over paper doily or stencil.