Start by beating the egg with a fork in a large mug. Then add the banana and mix without breaking the banana completely. --Tip: Prefer to use a wide, low mug rather than a tall, narrow mug, as mug cakes cook by exposure rather than depth.
Then add the oats (which can be oat flour, ground oats or even oat bran) and also the cocoa. Mix again with the fork.
Finally add the yeast, it will help your mug cake to rise.
Place in the microwave on full power for 2 minutes. At the end of that time, remove the cake and, if you see that the center is still raw, return it to the microwave for another 1 minute.
When your cake is ready, let it cool down a bit to make it easier to unmold. Then unmold and decorate to taste.