1 c. sugar
2 1/2 c. all-purpose flour (divided)
1 c. common sour cream
1 tsp. baking powder
1 tsp. vanilla
1/2 c. packed brown sugar
1 1/2 tbsp. cocoa
3/4 c. butter (divided)
2 eggs
1 tsp. baking soda
1 (6 oz.) pkg. semi-sweet chocolate pieces (divided)
1/2 c. chopped walnuts

Cream sugar and 1/2 cup butter until light and fluffy. Add 2 cups flour and next 5 ingredients. Beat at low speed with a mixer until blended. Increase speed to medium and beat 3 minutes, occasionally scraping bowl. Stir in 1/2 cup chocolate pieces. Spread batter evenly in greased 9-inch tube pan.

Combine 1/2 cup flour, brown sugar and cocoa. Cut in 1/4 cup butter until mixture resembles coarse crumbs and ingredients are well mixed. Stir in walnuts and remaining chocolate pieces. Crumble mixture evenly over top of batter. Bake at 350 degrees for 50-60 minutes or until coffee ring pulls away from sides of pan. Cool completely in pan on wire rack.