Preheat your oven to 375°F (190°C) and grease a muffin tin or line it with paper liners.

1. In a large bowl, whisk together the flour, baking powder, salt, and sugar.
2. In a separate bowl, whisk together the melted butter, milk, and egg. Add the wet ingredients to the dry ingredients and stir until just combined.
3. In a small bowl, mix together the cinnamon and brown sugar.
4. Spoon a tablespoon of the muffin batter into each muffin cup, then sprinkle a teaspoon of the cinnamon-sugar mixture on top. Add another spoonful of muffin batter on top of the sugar, followed by more cinnamon-sugar.
5. Use a toothpick or skewer to gently swirl the cinnamon-sugar mixture into the batter to create a swirl effect.
6. Bake for 15–20 minutes, or until a toothpick inserted into the center comes out clean.
7. While the muffins are baking, make the glaze by whisking together powdered sugar and a small amount of milk or cream until smooth.
8. Once the muffins have cooled slightly, drizzle the glaze over the tops.
9. Serve warm or at room temperature.