Prepare mug by coating the inside lightly with cooking spray. Pour all of the dry ingredients into a bowl.
Beat egg first with a spoon and mix in other liquid ingredients.
Then add dry ingredients and mix until you’ve removed all lumps. Pour the batter into the mug (do not fill more than halfway) and smooth the top with a spoon.
Thump mug firmly on the tabletop six times to remove excess air bubbles.
Place mug on top of a microwaveable small plate or saucer. Bake for 3-4 minutes. Check for doneness by inserting a toothpick in the middle of the microwave mug cake and removing the toothpick. If the toothpick is dry, the mug cake is done.