Chocolate-Hazelnut Cake

Preheat oven to 375 degrees F. Place hazelnuts in 9" by 9" metal baking pan. Bake 10 to 15 minutes until lightly toasted. Remove nuts from oven; turn oven control to 325 degrees F. To remove skin, wrap hot hazelnuts in clean cloth towel. With hands, roll hazelnuts back and forth until skins come off. In small saucepan over low heat, heat 3 squares semisweet chocolate with unsweetened chocolate, until melted and smooth, stirring frequently. Remove saucepan from heat. Set aside to cool. In food processor with knife blade attached, or in blender at medium speed, blend 1/2 cup toasted hazelnuts and 2 tablespoons sugar until very finely ground (reserve remaining hazelnuts for garnish). Blend in cake flour. Grease and flour 8-inch round cake pan. In large bowl, with mixer at low speed, beat margarine or butter and 1/3 cup sugar just until blended. Increase speed to high; beat until light and fluffy, about 1 minute. Reduce speed to low; beat in egg yolks, vanilla extract, and melted chocolate mixture until blended, about 1 minute, scraping bowl often with rubber spatula. In small bowl, with clean beaters and mixer at high speed, beat egg whites and cream of tartar until soft peaks form. Sprinkle in remaining 2 tablespoons sugar, beating until whites stand in stiff, glossy peaks. With rubber spatula, fold hazelnut mixture into yolk mixture just until blended. Gently fold beaten egg whites into yolk mixture, one-third at a time. Spread batter in pan. Bake 25 to 28 minutes until toothpick inserted in cake, about 2 inches from edge, comes out clean. (The center of cake will still be slightly soft.) Cool cake in pan on wire rack 15 minutes. With spatula, loosen cake from side of pan; invert onto wire rack to cool completely. (The recipe can be prepared up to this point 2 days in advance. If not using cake right away, wrap well with plastic wrap and refrigerate.) Coarsely chop remaining 3 squares semisweet chocolate; place in small bowl. In small saucepan over low heat, heat heavy or whipping cream just to boiling. Pour cream over chopped chocolate; let stand 1 minute. Gently stir until chocolate melts; stir in corn syrup. Let stand at room temperature until glaze begins to thicken, about 5 minutes. Meanwhile, coarsely chop reserved hazelnuts. Place cake on cake plate. Tuck strips of waxed paper under edge of cake to keep plate clean when glazing cake. Pour chocolate glaze over cake. With metal spatula, spread glaze to completely cover top and side of cake. Sprinkle chopped hazelnuts around top edge of cake. Chill cake 20 minutes to set glaze. Remove and discard waxed-paper strips. Refrigerate cake if not serving right away.