Grind the oatmeal finely to prepare.
Mash the banana and mix with the egg.
Add cocoa powder, oatmeal, and baking powder, then mix well.
Chop the diet chocolate into small pieces and add to the batter.
Line a baking pan with parchment paper and pour in the batter.
Bake in a preheated oven at 180°C for 20 minutes.
Let cool before serving.