In a microwave-safe mug, begin by whisking together the all-purpose flour, granulated sugar, baking powder, and salt until the mixture is uniform and free of lumps.
In a separate small bowl, combine the milk, vegetable oil, and vanilla extract. Gradually pour this wet mixture into the dry ingredients in the mug, stirring gently until everything is just combined; be careful not to overmix.
For the cinnamon swirl, take another small bowl and mix together the brown sugar and ground cinnamon. If you’d like some added texture, stir in the chopped walnuts at this stage.
Using a spoon, gently create small holes or a swirl in the batter of the mug cake. Spoon the cinnamon mixture into these holes, then carefully cover it with a bit more of the mug cake batter to encase the swirl.
Place the mug in the microwave and heat on high for about 1 minute to 1 minute and 20 seconds. Start checking it at the 1-minute mark; the cake should be cooked through but still moist. Be cautious of overcooking, as it may become rubbery.
After microwaving, allow the mug cake to cool for about a minute. Once slightly cooled, generously top the cake with cream cheese frosting, allowing it to drizzle delightfully over the warm cake.
Notes
Serve directly in the mug for a cozy touch, or transfer to a small plate. Garnish with cinnamon or chopped walnuts for an appealing finish.