Line a few trays with baking paper. I use 3 for this recipe as the cookies spread a lot.
Cream together the butter and sugars until light and fluffy.
Add the egg and stir in well.
Sieve in the flour, cocoa powder and salt and mix into a dough.
Pop in the fridge for 1 hour.
Preheat the oven to 180°C.
Remove from fridge and take a heaped tablespoon for each cookie. Roll into a ball and flatten into a disc. Put a heaped teaspoon of nutella in the centre of the disc and the banana (I put 4 pieces in). Roll the cookie back into a ball by pulling the sides in. Put on a baking tray, leaving a good distance between them as they will spread.
Bake for 10-12 mins until the cookies are firm to the touch on the edges and still soft in the centre.
Leave to cool slightly before enjoying.