In a microwave-safe mug, whisk together the flour, granulated sugar, baking powder, ground cinnamon, and salt until well combined.
Add the milk, melted butter, and vanilla extract to the dry ingredients. Stir until a smooth batter forms.
In a small bowl, combine the brown sugar and cinnamon. This will be your cinnamon swirl mixture.
Drop spoonfuls of the cinnamon swirl mixture onto the top of the cake batter in the mug. Use a toothpick or a fork to gently swirl it into the batter, creating a marbled effect.
Microwave the mug for about 1 minute and 30 seconds on high, checking after 1 minute. The cake should look set but still slightly moist in the center. Be careful not to overcook.
Allow the mug cake to cool for a minute before serving.
Notes
Top your cinnamon roll mug cake with a drizzle of icing made from powdered sugar and milk, or enjoy it with a scoop of vanilla ice cream for an extra treat!