PREHEAT oven to 350 °F. Grease and flour 12-inch Bundt pan or other tube pan.
COMBINE cake mix, pudding mix, milk, sour cream and eggs in large mixer bowl. Beat on low speed just until blended. Beat on high speed for 2 minutes. Stir in morsels and nuts. Pour into prepared baking pan.
BAKE for 55 to 65 minutes or until wooden pick inserted in cake comes out clean.
COOL in pan for 20 minutes. Invert onto wire rack to cool completely.
MELT baking bar and butter in small heavy-duty saucepan over low heat, stirring until smooth. Remove from heat. Stir in powdered sugar alternately with water until of desired consistency. Stir in vanilla extract.