Preheat oven to 375°F (190°C). Grease or line a 12-cup muffin tin.
In a large bowl, whisk flour, granulated sugar, baking powder, and salt.
In another bowl, whisk milk, oil, egg, and vanilla until combined.
Pour wet ingredients into dry and stir gently until just combined. Do not overmix.
In a small bowl, mix brown sugar and cinnamon.
Spoon 1 tablespoon of batter into each muffin cup.
Sprinkle 1 teaspoon cinnamon sugar mixture on top.
Repeat with remaining batter and cinnamon sugar.
Drizzle melted butter over each muffin.
Swirl gently with a toothpick to create a marbled effect.
Bake for 18–20 minutes or until a toothpick inserted in the center comes out clean.
Cool in the pan for 5 minutes, then transfer to a wire rack.