In a microwave-safe mug, combine the all-purpose flour, granulated sugar, baking powder, baking soda, ground cinnamon, and a pinch of salt. Whisk together until all dry ingredients are well blended.
Pour in the milk, vegetable oil, and vanilla extract. Use a fork or spoon to stir the mixture until the batter is smooth and free of lumps.
In a small bowl, combine the brown sugar and ground cinnamon to create the swirl mixture. Stir well to ensure an even blend.
Gently fold the cinnamon-sugar mixture into the mug cake batter. Use a knife or skewer to create a few swirls, allowing the cinnamon topping to mix lightly with the batter but not fully combine.
Place the mug in the microwave and cook on high for 1 minute. After this time, check the cake; it should have risen and look set. If it still appears wet in the center, microwave in additional 10-second intervals until completely cooked, taking care not to overcook as it may become dry.
Once fully cooked, let the mug cake cool for about 1 minute. If desired, drizzle with cream cheese frosting for a sweet finish.
Notes
Serve warm, optionally drizzled with cream cheese frosting.