In a large mug, melt the butter in the microwave for 15-30 seconds. Add in the brown sugar, egg yolk, and milk, and mix. Add the mashed banana and mix once again.
2 1/2 tbsp brown sugar, 1 yolk, 1 1/2 tbsp milk, 1 small banana, 1 1/2 tbsp butter
Add in the flour, salt, baking powder, and the walnuts, and stir just until combined. Microwave on high for 2 minutes and enjoy your mug cake.
If your banana mug cake still seems a bit raw, bake it for an additional 1o seconds and check it again.
Notes
When making mug cakes, make sure to enjoy them as soon as you make them. The longer they sit out the more the texture will change. So to enjoy it at its best, have it right out of the microwave :) (making sure to let it cool enough not to burn yourself of course!)