Mash the banana (I used an immersion blender, but you can also mash it vigorously with a fork) and mix it with the egg.
Mix the flour + baking powder and a pinch of salt into the banana-egg mixture, then add the blueberries. If the dough is not already in a bowl, pour it in now.
OVEN: Bake at 180°C/350˚F (conventional oven) for 20-25 minutes.
MICROWAVE: Bake for 140 seconds (2:20 minutes) at 800 watts.
Serve with a scoop of yogurt and some banana slices or extra blueberries if you like.
Tips
If you use certified gluten-free oat flour instead of whole wheat flour, this healthy mug cake is gluten-free.
I usually use 1 cup mugs for my mug cakes.
Double the recipe and you have two days' breakfast in a jiffy. Just reheat it the next day or eat it cold.
How to store
Cooled and covered with aluminum foil you can keep the mug cake in the fridge for 2 days (without any additional toppings)