Combine flour, baking powder and baking soda. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted; stir until chocolate is completely melted and mixture is well blended.
Add sugars; mix well. Blend in eggs, sour cream and vanilla.
Add flour mixture; mix well. Stir in nuts.
Spread into 9x5-inch loaf pan sprayed with cooking spray.
Bake 1 hour to 1 hour 10 min. or until toothpick inserted in center comes out clean. Cool bread in pan 10 min.