Preheat the oven to 180°C.
Smash the bananas with a fork.
Add oil, egg, and brown sugar to the bananas. Mix well. (wet ingredients)
In a separate bowl, mix together flour, baking soda, and cocoa powder. (dry ingredients)
Add the dry ingredients to the wet ingredient. Mix till just combined.
Add some chopped walnuts to the mix.
Spoon it to the muffin tin till ¾ full. If you like, add a slice of banana and chopped walnuts on top as decoration.
Bake in the oven for 15mins or until a skewer comes out clean.