In a microwave-safe mug (ideally around 12 oz), combine the all-purpose flour, granulated sugar, baking powder, and salt. Use a small whisk or fork to mix the dry ingredients thoroughly, ensuring an even distribution.
Next, pour in the milk, vegetable oil, and pure vanilla extract. Stir the mixture gently until it becomes smooth and creamy, ensuring there are no lumps remaining in the batter.
In a separate small bowl, blend the packed brown sugar with the ground cinnamon until evenly mixed. This will create your sweet cinnamon swirl.
Carefully dollop the cinnamon sugar mixture on top of your cake batter in the mug. Using a fork, gently swirl it through the batter in a circular motion, creating a beautiful marbled effect — be careful not to mix it in completely.
Place the mug in the microwave and cook on high for approximately 1 minute and 30 seconds. Keep an eye on the cake as it cooks, checking at the 1-minute mark. The cake should rise and be firm in the center when done. Cooking times may vary depending on your microwave's wattage, so adjust accordingly.
Once finished, allow the mug cake to rest for about 1 minute to cool slightly. If desired, top with a generous drizzle of cream cheese frosting or vanilla frosting for an extra indulgent touch.
Notes
Serve directly in the mug for a cozy feel. Optionally dust with powdered sugar or cinnamon before serving.