Melt butter and chocolate together. Let the cake cool. Preheat oven to 350 degrees and flour an 8-inch springform pan or spray with Pam. Beat egg yolks with 1/2 cup granulates sugar using a mixer with whisk attachment, beat until light and fluffy. Beat in the melted chocolate mixture.
Transfer mixture to another bowl. Clean out mixer bowl and whisk egg whites until foamy, then gradually add the 1/2 cup sugar, beating to form soft peaks. in 2 or 3 additions, fold egg whites into chocolate mixture. Pour mixture into prepared pan and bake 45 - 50 minutes with a shallow pan of water placed on the lowest rack in the oven.
When done, the cake will be firm to touch and the top covered with cracks. Cool on rack 10 to 20 minutes. The cake will deflate. Invert onto serving plate, sprinkle top with sifted powdered sugar. Serve warm or at room temperature. Yield: 16 servings.