Preheat oven to 375°F and line a muffin tin with paper liners.
In a medium bowl, whisk flour, baking powder, salt, and cinnamon.
In another bowl, whisk melted butter and brown sugar; add eggs, milk, vanilla.
Gently fold dry into wet ingredients; do not overmix.
Spoon batter into muffin cups ¾ full.
Bake 18–20 minutes, until tops are golden and a toothpick comes out clean.
Mix cinnamon and sugar. Brush muffin tops with butter and dip into topping.
Let muffins cool for 5 minutes in pan, then transfer to wire rack.