1. Heat the oven to 375 F. Line a 12-cup muffin tin with parchment paper liners or spray with baking spray.
2. Combine the flour, baking powder, soda, and salt in a large bowl. Mix and set aside.
3. In a mixing bowl, beat the room-temperature butter with the granulated sugar and brown sugar until light and fluffy.
4. Whisk the eggs with the milk, sour cream, and vanilla in a separate bowl until well blended. Add to the butter and sugar mixture, beating on low speed until blended.
5. Add the dry ingredients and stir with a wooden spoon or spatula just until the batter is moistened.
6. Scoop the batter into the muffin cups, filling about 3/4-full.
   

   

7. Bake the muffins for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs. Meanwhile, combine 3 tablespoons of granulated sugar and 1 1/2 teaspoons of cinnamon in a bowl. Set aside.
8. Let the muffins cool for 5 minutes, then turn them out onto a rack to cool.
   

   

 Make the Topping

1. While the muffins are still warm, brush them with the melted butter and then dip the tops in the cinnamon-sugar mixture.