Put butter into mug and microwave for 10 seconds to melt.
Add in egg and milk and lightly beat with a fork until combined.
Add mashed banana into the mug and continue stirring with you fork until mixed through.
Finally, add dry ingredients into the mug and beat with your fork until combined.
Place mug in the microwave and cook for 1 minute on medium heat.
The pudding should be risen and still slightly gooey in the middle. If still undercooked for your liking, cook it further in 10 second bursts in the microwave.
Serve with a generous scoop of coffee ice cream.