Whisk butter with sugar, brown sugar, vanilla and salt.
Smash banana with a fork. Add banana and egg to butter-mixture.
Sive flour and baking soda, mix in.
At the end mix in chopped chocolate and walnuts.
Form little "balls" using two teaspoons and place on baking tray, leaving enough space in between.
Bake in preheated oven at 180° C for 14 minutes.
Leave to cool on the backing tray.