Preheat oven to 170 C / Gas 3. Line 2 baking trays with baking parchment.
Cream the butter and sugar. Add the banana and vanilla, flour and salt and mix well. Fold in the chocolate.
Roll into walnut size balls and and place in the fridge for 15 minutes.
Flatten slightly and place cookies on the prepared baking sheet with ample distance between them. For a soft chewy cookie, bake for about 10 minutes. If you prefer them firmer, bake until lightly coloured for a few minutes longer.