In a microwave-safe mug, combine the all-purpose flour, granulated sugar, baking powder, 1/4 teaspoon cinnamon, and a pinch of salt. Mix well to ensure all dry ingredients are evenly distributed.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Stir until you have a smooth batter with no lumps.
In a small bowl, mix the brown sugar and 1/2 teaspoon cinnamon to create the cinnamon swirl mixture.
Spoon half of the batter into the mug, then sprinkle half of the cinnamon sugar mixture over it. Add the remaining batter on top.
Use a toothpick or knife to gently swirl the cinnamon sugar mixture into the batter, making a beautiful swirl pattern.
Microwave the mug cake for about 1 minute and 30 seconds, or until it has risen and is fully cooked (cooking times may vary based on your microwave).
Let it cool for a minute before adding a dollop of cream cheese frosting on top if desired.
Notes
Serve warm in the mug, topped with a drizzle of frosting or a light dusting of powdered sugar for added sweetness and appeal.