1. Heat oven to 425°F. Grease bottom of 9-inch springform pan.
2. Melt butter in medium microwave-safe bowl; add 3/4 cup cocoa and 1 cup sugar, stirring until well blended. Cool 5 minutes.
3. Stir in flour and vanilla; add egg yolks, beating well after each addition. Beat egg whites with remaining 1 tablespoon sugar in medium bowl until soft peaks form; gradually fold into chocolate mixture. Add chocolates. Spoon batter into prepared pan.
4. Bake 15 to 18 minutes or just until edges are firm (1-inch circle in center will be soft). With spatula, loosen cake from side of pan. Cool completely on wire rack; remove side of pan.
5. Cover; refrigerate at least 6 hours. Sift 2 teaspoons cocoa over whipped topping; stir until well blended. Garnish cake with chocolate topping. Cut cake while cold; garnish with additional chocolates. Makes 12 servings.