Chocolate Truffle Cake Supreme

Heat oven to 425°F (220°C). Grease bottom of 8-inch springform pan. In a medium saucepan, melt butter over low heat. Add cocoa and 1 cup sugar, stirring until well blended. Remove from heat; cool slightly. Stir in flour and vanilla. Add egg yolks, one at a time, beating well after each addition. In a medium bowl, beat egg whites with remaining 1 tablespoon sugar until soft peaks form; gradually fold into chocolate mixture. Stir in chocolate chips. Spoon batter into prepared pan. Bake 18 to 20 minutes or until edges are firm (center will be soft). Cool completely on wire rack (cake will sink slightly). Remove side of pan. Refrigerate cake for several hours. To Serve: Cut cake while cold, but allow to stand at room temperature for 10 to 15 minutes before serving. Serve wedges topped with sweetened whipped cream.