8 ounces bittersweet (not unsweetened) or semisweet chocolate, chopped
10 tablespoons (1 1/4 sticks) unsalted butter, cut into pieces
4 large eggs, separated
2/3 cup sugar
3 1/2 tablespoons all purpose flour
Preheat oven to 325F. Line bottom of 9-inch-diameter cake pan with 2-inch-high sides with waxed paper. Butter sides of pan and paper. Dust pan with flour. Melt chocolate and butter in top of double boiler over simmering water, stirring until smooth. Cool slightly. Whisk yolks and sugar in large bowl until pale yellow. Mix in flour, then chocolate mixture. Using electric mixer, beat whites in another bowl until stiff but not dry. Fold into chocolate mixture. Pour batter into prepared pan. Bake until toothpick inserted into center comes out with only a few moist crumbs attached, about 25 minutes. Cool completely in pan on rack. Run small sharp knife around sides of pan to loosen cake. Turn cake out onto platter and cool. (Can be prepared 8 hours ahead. Cover with cake dome and let stand at room temperature.) Sift powered sugar over cake and serve.