Step 1

Place the 2 tablespoons unsalted butter for the frosting in a small bowl and let sit at room temperature until softened.

Step 2

Place the 2 tablespoons unsalted butter for the cake in an 8-ounce or larger microwave-safe mug. Microwave until melted, 20 to 35 seconds.

Step 3

Add 2 tablespoons granulated sugar and 1 teaspoon vanilla extract and whisk with a fork to combine. Add 1/4 cup self-rising flour and mix just until you barely see any more flour. Add 3 tablespoons seltzer and stir until the batter is smooth.

Step 4

Microwave until the mug cake has risen and is firm and springy on top, 2 minutes 10 seconds to 2 minutes 50 seconds. Let cool while you make the frosting.

Step 5

Add 2 tablespoons of the powdered sugar, 1/8 teaspoon vanilla extract, and a pinch of kosher salt to the bowl of softened butter and stir to combine. Taste and add more powdered sugar as needed. Spoon the frosting on top of the mug cake.