PREHEAT oven to 350° F. Grease 13 x 9-inch baking pan.
COMBINE brown sugar, flour and butter in small bowl with pastry blender or two knives until mixture is crumbly. Stir in nuts and 1/2 cup morsels.
COMBINE flour, baking powder, baking soda and salt in small bowl. Beat granulated sugar, butter and vanilla extract in large mixer bowl until creamy. Add eggs, one at a time, beating well after each addition. Gradually add flour mixture alternately with sour cream. Fold in remaining morsels. Spread into prepared baking pan; sprinkle with topping.
BAKE 25 to 35 minutes or until wooden pick inserted in center comes out clean. Cool in pan on wire rack.