Mix butter and sugar with beater until fluffy. Melt chocolate and another stick butter. Add to sugar mixture and beat. Add eggs, and beat well. Add vanilla. Blend in flour.
Pour in buttered and floured waxed paper lined jelly roll pan. Spread mixture neatly all the way around. Bake at 350 degrees for 25-30 minutes. Cool for 10 minutes.
Unmold and pull paper off. Let cool; cut shape and layer in separate dishes or trifle bowl. Fill with your favorite chocolate mousse mixture. Chill at least 6 hours. Can be made 1 day ahead; top with whipped cream and chocolate sauce.
