Mash the banana in a microwave-safe mug. Then add all the ingredients (minus the oats and baking powder). Whisk the batter until combined.
Add the oats and baking powder and whisk again. Microwave for 1 minute and 30 seconds. Top with your favoriting toppings and enjoy!
Notes
Be sure to add the dry ingredients at the end. It will help the batter from bubbling over.
The batter will rise significantly (and then settle) during cooking, so make sure to use a mug with enough height.
Place a plate underneath the cup when microwaving to catch possible spills.
Don't overcook! The exact time will differ slightly as every microwave is different. Start checking after 90 seconds and add 10 or so more seconds for tall mugs. It should be soft and moist with a little bit of moisture, which is good! It will continue to cook as it cools and will firm up.