Add all dry ingredients together in small bowl and mix well.
Add mashed banana, egg whites, milk and sweetener to dry ingredients and mix well. Add to mug.
Cook for 30 seconds in microwave and mix well. Cook an additional 30 seconds. If cake still appears wet, cook for 25-30 seconds increments until done.
Top with nut butter and cocoa nibs if desired.
Note: Cooking at a lower power (50%) yields a more gooey and tender cake. You will have to cook the mug cake longer. Keep an eye on cakes as they cook as they can expand a lot and overflow if mug is not large enough.