Preheat oven to 325°F. Butter and flour 10 inch diameter springform pan with 2 3/4 inch sides. Stir chocolate and butter in heavy medium saucepan over low heat until chocolate melts and mixture is smooth. Beat eggs and 1 1/4 cups sugar in large bowl until well blended and beginning to thicken. Sift flour and baking powder over eggs and fold in. Gradually fold in chocolate mixture. Transfer batter to prepared pan.

Bake cake 20 minutes. Cover pan with foil. Bake until tester inserted into center comes out with moist crumbs still attached, about 30 minutes longer. Uncover cake; cool in pan on rack. Cover; let stand at room temperature.

Cut around pan sides to loosen cake. Release pan sides. Sift powdered sugar over cake; cut into wedges.