Preheat your oven to 375°F (190°C) and grease a muffin tin.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, mix the milk, melted butter, vanilla extract, and eggs.
Combine the wet ingredients with the dry ingredients and mix until just combined.
In a small bowl, mix together the brown sugar, cinnamon, and melted butter for the filling.
Fill each muffin cup halfway with batter, add a spoonful of the cinnamon sugar filling, then top with more batter.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
While the muffins are baking, prepare the glaze by mixing the powdered sugar, milk, and vanilla extract.
Once the muffins are done, let them cool for a few minutes before drizzling the glaze on top.