Preheat the oven to 180 C / Gas 4. Line baking tray with parchment.
Beat the butter and sugar together until pale and fluffy, then gradually beat in the mashed banana, then the egg. Sift in the flour, cinnamon and salt, then fold in with the oats and sultanas.
Drop heaped teaspoonfuls of the mixture onto baking trays, leaving enough space around each cookie to allow it to spread.
Bake for 10–15 minutes or until golden brown. Cool slightly on the baking trays, then transfer to a wire rack and leave to cool completely. These cookies can be kept in an airtight container for 3–4 days.