Mix together cake ingredients in a mug. Microwave for 45-60 seconds depending on your microwave. Check your cake with a toothpick to see if it's done.
While the cake cools, make your frosting. If your butter is cold microwave it for 5 seconds. Whip it really well with a fork. When whipped add powdered sugar and whip until incorporated. Add vanilla, salt, and if needed milk. Mix again. Spoon over cooled (or at least mostly cooled) cake.