01 - Preheat the oven to 350°F. Grease a 24-cup mini muffin tin with cooking spray or line with paper liners.
02 - In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon until well combined.
03 - In a separate medium bowl, whisk together the milk, vegetable oil, egg, and vanilla extract until smooth.
04 - Pour the wet ingredients into the dry ingredients. Gently fold together with a spatula until just combined. The batter should remain slightly lumpy—avoid overmixing.
05 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
06 - Bake for 10 to 12 minutes, or until a toothpick inserted into the center of a muffin comes out clean. The tops should be lightly golden.
07 - Let the muffins cool in the pan for 5 minutes, then carefully transfer to a wire cooling rack.
08 - While the muffins are still warm, dip the top of each muffin into the melted butter, then immediately roll in the cinnamon sugar mixture to coat evenly.
09 - Serve warm or at room temperature. Best enjoyed the same day for optimal texture.