Preheat oven to 325 degrees.
Separate 12 eggs.
Place egg yolks and 11/2 cups sugar in small mixer bowl; beat for 15 minutes until thickened and lemon col- ored.
Add walnuts and vanilla.
Beat egg whites until stiff peaks form; fold into yolk mixture.
Pour batter into 3 springform pans or 9-inch cake pans with scraper.
Bake for 1 hour or until cake tests done.
Remove pans from oven; invert onto cake racks until cool.
Remove cake from pans.
Place butter, choco- late and remaining sugar in small bowl; beat until blended.
Add remaining eggs; beat until of spreading consistency.
Spread on tops of layers and on sides; as- semble cake.
Sprinkle top with additional ground walnuts.