Preheat the oven to 170 ° C and line a baking sheet with parchment paper. Beat the butter with both types of sugar until it is creamy. Gradually stir in the eggs, about 30 seconds at a time. Sift the flour with the baking powder over it and stir in with the salt.
Peel the banana, mash it finely with a fork and stir in with the oat flakes and nuts and stir to a smooth batter. Place small heaps with 2 teaspoons, a little space apart, on the baking sheet, flatten slightly and bake in the oven for 11-14 minutes until golden brown.
Remove from the baking sheet and let cool down on a wire rack. If you want to beautify it a bit, you can melt the couverture in a water bath and cover the biscuits with it as you wish.
If you put piles on the baking sheet, you will have cookies that are a little softer in the middle because they don`t really diverge. If you smooth the dough, however, you will have wonderfully crispy cookies all around.