Mash 1/2 banana with fork in a small bowl, microwave 30 seconds. DON'T skip this step!
Add all ingredients to banana and mix until well blended.
Spray inside of regular-size mug with Pam. Dump mix into mug.
Microwave on high until the cake is towering over the edge of the mug (approx. 2 minutes). Open the microwave door to stop the cooking, and then close door and microwave again for approx. 30 seconds, until the cake is towering again. (The cake will fall back into the mug each time the cooking stops.) Let it sit for 2-3 minutes. Top with your favorite toppings! Mmmm!