In large mixer bowl, soften butter (10 sec).
Add cake mix, water and eggs.
Blend and beat as directed on package.
Place 2 cups of batter in another large mixing bowl.
To this batter add 1 cup of dry frosting mix and chocolate; mix well.
Spoon this mixture into a swiss roll dish, bake until done. Allow to cool.
Apply a even layer of frosting and roll up. Cut into slices and serve.