Heat oven to 350º. Grease and flour jelly roll pan. Prepare cake mix as directed on package. Pour batter into pan. Mix remaining ingredients. Drop by tablespoonfuls onto batter. Cut through batter with knife several times for marbled effect. Sprinkle with additional chips and chopped nuts if desired. Bake until cake springs back when touched lightly in center, or when wooden pick inserted in center comes out clean. Bake 25-30 minutes.