Place egg whites in a large bowl; let stand at room temperature 30 minutes. Preheat oven to 350°. In top of a double boiler or a metal bowl over barely simmering water, melt chocolate and butter; stir until smooth. Remove from heat; cool slightly., In another large bowl, beat egg yolks until thick and lemon-colored. Beat in chocolate mixture. With clean beaters, beat egg whites and cream of tartar on medium speed until foamy., Gradually add sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Continue beating until stiff glossy peaks form. Fold a fourth of the egg whites into chocolate mixture, then fold in remaining whites., Transfer to a greased 9-in. springform pan. Bake until a toothpick inserted in center comes out with moist crumbs, 40-45 minutes (do not overbake). Cool completely on a wire rack., Loosen sides from pan with a knife. Remove rim from pan. If desired, dust with confectioners' sugar.