Preheat oven to 375°.
To prepare batter, lightly spoon the flours into dry measuring cups; level with a knife. Place flours in a large bowl. Stir in oat bran and next 6 ingredients (through salt); make a well in center of dry ingredients. Combine sour cream and next 5 ingredients (through egg); stir well with a whisk. Add to dry ingredients; stir just until moist. Spoon mixture evenly into 12 muffin cups coated with cooking spray.
To prepare topping, combine 1/2 cup dark brown sugar and remaining ingredients in a small bowl; toss with a fork until moist. Sprinkle topping evenly over batter. Bake at 375° for 15 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack.