In a small bowl, whisk the egg, melted butter, water, and vanilla extract.
Whisk in dry ingredients: gluten-free flour, sugar, baking powder, and sugar until smooth and no lumps show.
Pour the batter into a 12-ounce coffee mug.
Microwave at 900W for 90 seconds in the center of the microwave or until a pick inserted in the center of the mug cake comes out clean and dry. If wet, return in 10-second bursts.
Cool down for a few minutes in the mug, then top with your favorite toppings: whipped cream, sprinkles, berries, etc.
Don't store leftovers! Eat them immediately.