4 ounces fine-quality bittersweet chocolate (not unsweetened)
1/2 cup milk chocolate chocolate chips
1 stick (1/2 cup) unsalted butter
3/4 cup sugar
5 large eggs, separated
3/4 unsweetened cocoa powder
Preheat oven to 375°F and butter an 9-inch round baking pan. Line bottom with a round of wax paper and butter paper.
Chop chocolate into small pieces. In a double boiler or metal bowl set over a saucepan of barely simmering water melt chocolate with butter, stirring, until smooth. Remove top of double boiler or bowl from heat and whisk sugar into chocolate mixture. Add eggs yolks and whisk well. Sift cocoa powder over chocolate mixture and whisk until just combined. In a separate bowl, beat egg whites until soft peaks form; fold these into chocolate mixture. Pour batter into pan and bake in middle of oven 25 minutes, or until top has formed a thin crust. Cool cake in pan on a rack 5 minutes and invert onto a serving plate.
Dust cake with powdered sugar and serve!