Combine all of the cake ingredients in a microwave-safe mug, dish, or ramekin that is at least eight ounces. Stir until everything is well incorporated.
To make the cinnamon swirl, melt half a tablespoon of butter in a small bowl. Add cinnamon and brown sugar, and stir until combined.
1 ½ tbsp brown sugar, ½ tbsp butter, ¼ tsp cinnamon
Pour the cinnamon swirl mixture over the cake batter and use a butter knife to fold and swirl it into the batter.
Microwave the cake for one minute or until the center is cooked through.
While the cake cools, make the frosting by adding butter and cream cheese to a small bowl. Heat the mixture at 50% power in the microwave for 10 seconds, then add four tablespoons of powdered sugar and stir until smooth. Add up to an additional tablespoon of powdered sugar to reach the desired frosting consistency.
Frost the cake by spooning on the desired amount of frosting or creating a swirl pattern as shown. To create the swirl, transfer the frosting to a small sandwich bag, snip off a corner, and pipe the frosting into a swirl, starting at the center of the cake. Enjoy!
Notes
This frosting recipe is sufficient for two servings if you use the suggested swirl pattern. If you want to share your cake with someone else, you can make another cake using the same recipe. Alternatively, you can refrigerate any leftover frosting for later use.