Cinnamon Swirl Muffins

Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease the cups thoroughly with butter or non-stick cooking spray.
In a large mixing bowl, whisk together the 2 cups of flour, 3/4 cup of sugar, baking powder, and salt until they are well combined.
In a separate medium bowl, whisk the slightly cooled, melted butter with the egg, milk, and vanilla extract until the mixture is smooth and fully incorporated.
Pour the wet ingredients into the dry ingredients and stir with a spatula until the batter is just combined and no dry streaks of flour remain; be careful not to overmix, as the batter will be slightly lumpy.
In a small bowl, mix the 1/3 cup of sugar with the cinnamon to create the swirl filling.
Spoon about 1 tablespoon of batter into the bottom of each prepared muffin cup. Sprinkle a generous teaspoon of the cinnamon-sugar mixture over the batter, then top with the remaining batter, dividing it evenly among the cups.
Brush the tops of the muffins with the 1/4 cup of melted butter and sprinkle with any remaining cinnamon-sugar mixture.
Bake for 18-22 minutes, or until the muffins are golden brown and a toothpick inserted into the center comes out clean. Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.