Start by greasing your mug lightly with butter or oil. This makes cleanup easier and helps the cake rise evenly.
In the mug, combine the flour, baking powder, sugar, and salt. Mix with a fork to distribute everything evenly.
Add the milk, melted butter, and vanilla. Stir until you get a smooth batter—don't overmix or it may turn rubbery.
In a separate small bowl, combine brown sugar, cinnamon, and softened butter. This is your gooey cinnamon swirl.
Spoon the swirl mixture on top of the batter and gently swirl it using a knife or spoon. Make sure you don’t mix too much—those swirls are golden!
Microwave on high for 1 minute and 30 seconds. Check if it’s done by gently pressing the top; it should spring back. If not, add 10 more seconds.
While the mug cake cools slightly, mix your icing ingredients in a small bowl until smooth. Pour it over the warm cake and let it melt slightly.
Dig in and enjoy the rich cinnamon flavor and tender crumb. It’s cozy comfort food in a cup!