Place the peeled banana in a large microwave-safe mug (16-ounce or larger) and use a fork to mash until smooth.
Add in the egg, peanut butter, maple syrup, flour, baking powder, and cinnamon, and stir well. Sprinkle a few chocolate chips or walnuts on top, if you'd like extra texture.
Place the mug in the microwave and cook on high power for 2 minutes. The cake will rise above the top of the mug as it cooks, but it shouldn't spill over as long as you use a 16-ounce or larger size mug. The cake should feel firm to the touch in the center when it's done cooking; add another 30 seconds of cook time if you need to.
Carefully remove the mug from the microwave (it will be hot!) and let it cool for 5 minutes before digging in.
Notes
Nutrition information is for 1 large serving without the optional toppings. This information is automatically calculated and is just an estimate, not a guarantee.Mug Cake for Two: If you want to make a snack-size portion or have smaller mugs to work with, divide this batter into two 10-ounce coffee mugs and cook them simultaneously for 2 minutes in the microwave.Update Note: This recipe was updated in July 2023 to use a microwave and turn out more like banana bread. Find the original recipe here if you prefer the flourless oven-baked version, which does not work in a microwave.