In ramekin or mug, combine mashed banana, egg, and coconut flour.
Microwave on high for 1-2 minutes until cake tests done. Time will vary depending on wattage of microwave.
Carefully remove from microwave and set on a rack to cool. Then enjoy from microwave dish or remove to serve.
Notes
Overcooking mug cakes result in a rubbery texture. So take the cake out when it's a little gooey in the middle.A medium banana should weigh about 60 grams so you may need a little more or less depending on the size.