Combine 2 cups sugar, 3/4 cup water and 1/4 cup corn syrup in heavy medium saucepan. Stir over low heat until sugar dissolves. Increase heat and boil without stirring until candy thermometer registers 234 to 240 degrees F(soft ball stage).
Meanwhile, using a heavy-duty electric mixer, beat 6 egg whites with 3/4 cup sugar in large bowl until soft peaks form. Beat in hot syrup in slow steady stream. Beat on medium speed until egg white mixture cools to room temperature, about 20 minutes.
Cream 2 cups plus 2 tablespoons (4 1/2 sticks) butter in medium bowl until light and fluffy. Gradually beat into egg white mixture (mixture may appear curdled). Continue beating until smooth, light and fluffy.
Place 2/3 pound chocolate in top of double broiler over barely simmering water. Turn off heat and stir until chocolate melts. Cool slightly. Using low speed, gradually beat chocolate into buttercream. Mix in 1 1/2 teasps vanilla. Transfer to large bowl.
Repeat process 2 more times with remaining ingredients. (Can be prepared 2 days ahead. Cover and refrigerate. Bring to room temperature before beating slightly to restore mixture.).
Makes enough to frost and decorate a 6", 9" and 12", three tier wedding cake.