In a large mixing bowl, combine the dry ingredients and set aside.
In another bowl whisk together the melted butter, egg and sour cream.
Add butter, egg and sour cream mixture to the dry ingredients, and fold together until just combined.
Fold in HALF of the streusel topping to the batter.
Divide the batter between the muffin cups.
Top muffins with remaining streusel topping.
Bake for 20-25 minutes.
Remove from oven when a toothpick inserted in the middle of a muffin comes out clean.
Cool completely on wire racks.
Muffins can be frozen after baking.Wrap each muffin tightly with foil and wrap with plastic wrap. Freeze. To eat, remove from freezer and thaw on the counter until soft.Alternatively, remove the foil and plastic wrap, and blast in the microwave for 30 seconds.How to store fresh baked muffinsMake sure they are completely cooled before storing in a covered container at room temperature for up to 3 days.Substitutions/AdditionsGreek yogurt can be used in place of sour cream.You could sub the walnuts for pecans or almonds in the streusel topping if that's what you have.