Preheat the oven to 375°F (190°C) and grease your muffin tin.

In a mixing bowl, whisk together flour, sugar, baking powder, and salt.

In a separate bowl, mix milk, melted butter, egg, and vanilla until smooth.

Pour wet mixture into dry ingredients and fold gently until combined.

In a small bowl, mix brown sugar, cinnamon, and melted butter for the filling.

Fill muffin cups halfway with batter, add a spoonful of cinnamon filling, then top with more batter.

Swirl the filling through the batter in each muffin cup using a toothpick.

Bake for 15-18 minutes until golden brown and a toothpick comes out clean.

Let muffins cool slightly, then optionally glaze with a mixture of cream cheese, powdered sugar, and milk.