Preheat the oven to 375°F (190°C) and line a muffin tin with paper liners.
The flour, baking powder, and salt should be whisked together in a medium bowl. Set aside.
Mix the butter, brown sugar, and granulated sugar together in a large mixing bowl until light and fluffy. This should take about 2-3 minutes.
Add egg, mixing well after each addition. Stir in the vanilla extract.
As you add the dry ingredients, alternate with the milk. Using the dry ingredients as a starting point and ending point, mix until combined. Don’t overmix the batter.
Spoon the batter into the prepared muffin tin, filling each liner about three-quarters full.
Bake the muffins for 15-18 minutes, or until a toothpick inserted into the center comes out clean. After removing the muffins from the tin, cool them on a wire rack for about 10 minutes.
While the muffins are still warm, prepare the cinnamon sugar topping. In a shallow bowl, combine the granulated sugar and ground cinnamon.
Dip each cooled muffin into the melted butter, ensuring it is fully coated. Then roll the muffin in the cinnamon sugar mixture, gently pressing it to adhere.
Place the coated muffins on a serving plate or tray and enjoy!