In a small mixing bowl, add all your ingredients, except for the powdered sugar, and mix well. If the batter is too thick, add extra milk.
Grease a microwave safe mug or bowl generously and pour the batter in it and microwave for 1 minute. If the cake is undercook, microwave in 20 second bursts until done. Remove from the microwave and let sit for one minute.
While the mug cake is cooling, make your frosting by whisking the powdered sugar with water until desired texture is achieved. Drizzle over the mug cake.
Follow as above, but bake in an oven safe dish at 180C/350F for 10 minutes.