Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, mix together the flour, ¾ cup sugar, baking powder, and salt.
In a separate bowl, whisk together the milk, vegetable oil, and egg. Add this to the dry ingredients, stirring just until moistened.
Fill each muffin cup about ⅔ full with the batter.
In a small bowl, combine ⅓ cup sugar, cinnamon, and melted butter for the cinnamon swirl. Drop a teaspoon of this mixture onto the top of each muffin batter.
Use a toothpick to swirl the cinnamon mixture into the batter.
Bake for 20-25 minutes or until a toothpick inserted into a muffin comes out clean.
While the muffins are baking, prepare the glaze by mixing the powdered sugar and milk until smooth.
Once the muffins are done, cool them for a few minutes in the pan, then transfer to a wire rack.
Drizzle the glaze over the warm muffins.
