Preheat oven to 350°F.
Topping
Whisk together the dry topping ingredients (without the butter), then divide into 2 portions. Set aside.
Muffins
In a medium mixing bowl, whisk together flours, baking powder and salt. In an electric mixer bowl, mix together melted butter, sugar, eggs and milk until blended. Add flour mixture to butter mixture and mix again until smooth and well-blended. Add sour cream and mix again just until blended. Divide batter into two equal portions.
Butter (or spray with non-stick spray) the cups of a 6-large-cup muffin pan. Pour equal portions of one-half of batter into each muffin cup. Sprinkle equal portions of 1/2 of the topping mix over the batter in each muffin cup. Drizzle equal portions of 1/2 of the melted butter over the topping mixture. Pour remaining batter in equal portions into each cup and repeat process with remaining topping and melted butter.
Bake in oven for 25-30 minutes, or until a toothpick inserted into center comes out clean.. Allow to cool for a few minutes, then either serve warm or transfer to a cooling rack.