Preheat oven to 180°C (350°F) and line a muffin tray.
In a bowl, mix flour, sugar, baking powder, and salt.
In another bowl, whisk eggs, melted butter, milk, and vanilla.
Combine wet and dry ingredients until smooth.
In a small bowl, mix brown sugar, cinnamon, and softened butter.
Fill muffin cups halfway with batter.
Add a spoon of cinnamon mixture, then top with more batter.
Swirl gently with a toothpick.
Bake for 18–20 minutes until golden.
Mix powdered sugar and milk, then drizzle over cooled muffins.