I used this recipe for inspiration, but adapted it, as reflected in the image. It still turned out too sweet for me, so I would try using less agave or maple, and you could probably even replace it with your sweetener of choice and just increase the milk by a tablespoon. Before warned that you should use a large mug because mine overflowed.
https://www.thehealthyhomecook.com/powder-pb-banana-mug-cake/
by leidance