In a microwave-safe mug, mix together the flour, granulated sugar, baking powder, cinnamon, and salt until well combined.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Stir until the batter is smooth and there are no lumps.
In a small bowl, mix the brown sugar and ground cinnamon until well combined.
Spoon half of the cake batter into the mug, then sprinkle the cinnamon-sugar mixture on top. Pour the remaining batter over it, sealing the filling inside.
Microwave the mug cake for 1 minute and 30 seconds, or until it has risen and a toothpick inserted in the center comes out clean. Cooking time may vary depending on your microwave, so check for doneness.
If desired, prepare the cream cheese frosting by mixing the cream cheese, powdered sugar, and milk together until smooth and creamy.
Top the warm mug cake with the cream cheese frosting and enjoy immediately!
Notes
Optional cream cheese frosting adds a delicious touch.