In heavy saucepan, heat chocolate, butter, and water over medium heat, stirring till blended.
Add egg yolks, sugar, and vanilla; stir till smooth.
Chill till mixture is of spreading consistency, about 45 minutes.
Slice cake horizontally in 6 layers.
Spread chocolate between layers, then frost top and sides.
Chill at least 45 minutes before serving.
Cut in 1/4 inch slices.