Begin by selecting a microwave-safe mug and adding the all-purpose flour, granulated sugar, baking powder, cinnamon, and salt. Stir these dry ingredients together thoroughly until they are evenly blended.
Pour in the milk, vegetable oil, and vanilla extract into the mixture. Stir gently but thoroughly until the batter is smooth, ensuring there are no lumps remaining.
In a separate small bowl, combine the brown sugar and ground cinnamon to create the cinnamon-sugar filling. Mix until the ingredients are fully incorporated.
Take your prepared mug batter and spoon half of it into the mug. Evenly sprinkle the cinnamon-sugar mixture on top of this layer. Carefully pour the remaining batter over the filling to secure it within the cake.
Place the mug in the microwave and cook for about 1 minute and 30 seconds. The cake should rise and appear firm. To check for doneness, insert a toothpick into the center; it should come out clean. Cooking times may differ based on microwave wattage, so keep an eye on it!
If you're in the mood for frosting, mix together cream cheese, powdered sugar, and milk in a small bowl until the mixture is smooth and creamy.
Once the mug cake is done and slightly cooled, top it with the cream cheese frosting if desired. Indulge while it's warm for the best experience!
Notes
Consider garnishing the mug cake with a sprinkle of additional cinnamon or a drizzle of honey for an extra sweet touch! Enjoy with a hot cup of coffee or tea!