Preheat the oven to 180°C (356°F).
Mash the banana in a bowl.
Add eggs, cocoa powder, protein powder, and baking powder to the mashed banana and mix well.
Add nuts to complete the batter.
Pour the batter into a pan lined with parchment paper.
Bake in the oven at 180°C (356°F) for 20 minutes.
Let it cool and cut into pieces.