Place the dry ingredients into a microwave safe mug or jar. Use a fork to blend.
Add the mayonnaise, egg, and water, stirring to blend thoroughly. Allow the batter to rest for 2 minutes.
Microwave for 50 seconds. Serve with sugar-free chocolate syrup, sugar-free caramel syrup, whipped topping, and/or chopped nuts.
This mug cake is so good!