Add all ingredients except mini marshmallows into a microwave-safe mug. Mix with a small whisk until batter is smooth. Add in mini marshmallows. If you want them on the top, leave them as is. If you want them mixed into the batter, stir them into the batter.
Microwave for about 1 minute, or until cake is cooked. Top of cake should be dry to the touch. Cake is best consumed warm.
