Preheat the oven to 400°F. Line a 12-count muffin tin with paper liners or grease it well.
Whisk together the flour, baking powder, baking soda, and salt in a medium bowl.
In a large bowl, stir together the melted butter, granulated sugar, and brown sugar. Then whisk in the eggs, milk, and vanilla.
Gently mix the dry ingredients into the wet, stirring just until combined.
Fold in the cinnamon. Then pour the batter into the muffin tin, filling each cup most of the way full.
Sprinkle the tops with cinnamon sugar. Then bake at 400°F for 5 minutes. Reduce the oven temperature to 350°F and bake for another 14-17 minutes, until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool for a few minutes in the pan. Remove them from the pan and onto a wire rack to cool completely.