Preheat oven to 350F. Grease and flour 2 (9-inch) round cake pans. Cover bottoms of pans with wax paper. Grease and flour wax paper; set aside. Microwave chocolate and water in medium microwaveable bowl on HIGH 1 to 2 min. or until chocolate is almost melted, stirring after each min. Stir until chocolate is completely melted; cool completely. Stir in sour cream.
Mix flour, baking powder, baking soda and salt; set aside. Beat butter and sugars in large bowl with electric mixer on medium speed until light and fluffy.
Add eggs, 1 at a time, beating well after each addition.
Add flour mixture alternately with the chocolate mixture, beating after each addition until well blended.
Add vanilla; mix well.
Pour evenly into prepared pans.
Bake 35 to 40 min. or until toothpick inserted in centers comes out clean. Cool cakes in pans 10 min.; remove from pans. Cool completely on wire racks. Fill and frost with BAKER'S ONE BOWL Chocolate Frosting.