In a microwave-safe mug, whisk together the flour, granulated sugar, baking powder, and salt until well combined.
In a separate bowl, mix the milk, vegetable oil, and vanilla extract. Slowly pour this wet mixture into the dry ingredients, stirring until just combined.
In a small bowl, make the cinnamon swirl by mixing the brown sugar and ground cinnamon. If desired, add chopped walnuts for extra crunch.
Gently poke holes or create a swirl in the mug cake batter and spoon in the cinnamon mixture, then cover it with the batter.
Microwave the mug on high for about 1 minute to 1 minute and 20 seconds, checking at the 1-minute mark. You want it cooked, but still moist; avoid overcooking as it will harden.
Once done, let the mug cake cool for a minute before topping it with cream cheese frosting.
Notes
For extra flavor, use homemade cream cheese frosting.