Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease with non-stick spray.
Mix dry ingredients: In a large mixing bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
Cream butter and sugar: In another bowl, beat softened butter and sugar until light and fluffy using a hand mixer or stand mixer.
Add eggs and vanilla: Mix in eggs one at a time, then stir in vanilla extract.
Combine wet and dry: Slowly add the dry ingredients to the wet mixture, alternating with milk. Stir gently until just combined (do not overmix).
Fill muffin tin: Scoop batter evenly into prepared muffin cups, filling each about ¾ full.
Prepare topping: In a small bowl, mix sugar and cinnamon. Brush muffin tops lightly with melted butter, then sprinkle with the cinnamon-sugar mixture.
Bake: Place in the oven and bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean.
Cool muffins: Let muffins rest in the tin for 5 minutes, then transfer to a wire rack to cool completely.
Lightly sprinkle powdered sugar over cooled muffins before serving.