Preheat the oven to 180°C (356°F).
Mash the banana well.
Mix almond flour, cocoa powder, protein powder, and baking powder.
Add the eggs and mashed banana, then mix well.
Add chopped walnuts and mix again.
Pour the batter into an oven-safe pan.
Bake in the preheated oven for 20 minutes.
Let it cool, then cut into bite-sized pieces.