Add the melted butter, and mix until large crumbs form.
Set aside while you make the muffins.
To make the muffins:
Preheat oven to 375°F (190°C). Grease or line 12 standard muffin cups.
Combine the flour, sugar, baking powder, baking soda, and salt in a large bowl. Make a well in the center of the mixture.
In a separate bowl, combine the cooled butter, sour cream, eggs, and vanilla. Add to the well in the flour mixture. Stir just until combined. Do not overmix.
Divide the batter among the prepared muffin cups, using about 3 tablespoons of batter per cup.
Sprinkle the crumb mixture over the tops of the muffins, pressing it down slightly into the batter.
Bake 20-24 minutes, or until the tops of the muffins are golden brown, and a pick inserted into the center comes out clean.
Cool the muffins in the pan for 10 minutes. Then transfer the muffins to a wire rack to cool completely.
Notes
*Add 1 teaspoon of cinnamon to the crumb topping if you like.