Preheat oven to 300 degrees F. Remove bottom from 9" by 3" springform pan and cover with foil, wrapping foil around to the underside (this will make it easier to remove cake from pan). Replace bottom. Grease and flour foil bottom and side of pan. In large glass bowl, combine butter and both kinds of chocolate. In microwave oven, cook, uncovered, on Medium (50%), 2 1/2 minutes; stir. Return chocolate mixture to microwave oven; cook 2 to 2 1/2 minutes longer until almost melted; stir until smooth. (Or, in heavy 2-quart saucepan, heat butter and both kinds of chocolate over low heat until melted, stirring frequently. Pour chocolate mixture into large bowl.) In small bowl, with mixer at high speed, beat egg yolks and sugar until very thick and lemon-colored, about 5 minutes. Add egg-yolk mixture to chocolate mixture, stirring with rubber spatula until blended. In another large bowl, with clean beaters, and with mixer at high speed, beat egg whites and cream of tartar until soft peaks form. With rubber spatula or wire whisk, gently fold beaten egg whites into chocolate mixture, one-third at a time. Pour batter into pan, spreading evenly. Bake 35 minutes. (Do not overbake. The cake will firm upon standing and chilling.) Cool cake completely in pan on wire rack. Then, refrigerate overnight in pan. To remove cake from pan, run a hot knife around edge of cake, then lift off side of pan. Invert cake onto cake plate; unwrap foil from bottom and lift off bottom of pan. Carefully peel foil from cake. Let cake stand 1 hour at room temperature before serving. Just before serving, decorate cake: Sprinkle confectioners' sugar through fine sieve over star stencil for a pretty design (see Tips & Techniques). Or, dust top of cake with confectioners' sugar. Each serving without confectioners' sugar: About 200 calories, 4 g protein, 17 g carbohydrate, 14 g total fat (6 g saturated), 100 mg cholesterol, 110 mg sodium.
