Preheat the oven to 180°C (356°F).
Mash the banana with a fork and set aside.
In a bowl, combine the mashed banana, egg, low-calorie sugar, and vanilla extract, and mix well.
Add cocoa powder, almond flour, baking powder, and salt, and gently mix.
Add chopped nuts and fold evenly.
Pour the batter into a baking pan and smooth the top.
Bake in the oven at 180°C (356°F) for 20-25 minutes.
Let it cool, then cut into desired sizes.