For the Muffins

1. Heat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Line 8 cups of a standard 12-cup muffin tin with paper liners.
2. Put the flour, baking powder, salt, white sugar, and brown sugar into a large mixing bowl. Whisk these dry ingredients together until they are well combined.
3. In a separate, large measuring cup, pour in the milk and vegetable oil. Add the egg to this cup. Whisk the wet ingredients together until they are fully mixed.
4. Pour the wet ingredients into the bowl with the dry ingredients. Stir the mixture gently just until you no longer see any streaks of dry flour. Avoid overmixing the batter.

For the Cinnamon Sugar Swirl

1. In a small bowl, mix together the granulated sugar and ground cinnamon until combined. This is your cinnamon sugar topping mixture.
2. Place one tablespoon of the muffin batter into the bottom of each of the 8 prepared muffin cups. Sprinkle a small amount of the cinnamon sugar mixture over the batter in each cup.
3. Add another tablespoon of muffin batter into each cup on top of the cinnamon sugar layer. Sprinkle more of the cinnamon sugar mixture over this second layer of batter.
4. Keep adding batter and cinnamon sugar layers alternately until each muffin cup is about three-quarters full. Finish by topping the batter in each cup with the rest of the cinnamon sugar mixture.

Baking

1. Bake the muffins in the preheated oven at 350 degrees Fahrenheit (175 degrees Celsius) for about 20 to 25 minutes. They are done when the tops look golden brown and a toothpick inserted into the center of one muffin comes out clean, without wet batter attached.
2. Let the muffins cool down completely inside their muffin tin before you take them out and serve them.