Preheat the oven to 180C/160C Fan/Gas 5. Line 2 large baking trays.
In a large bowl, or the bowl of a stand mixer, combine the butter, both types of sugar, the banana and the syrup. Mix until combined - it may look a little lumpy but this is fine!
Fold in the flour, oats, baking powder and baking soda. Use a tablespoon to drop balls of the cookie dough onto the prepared baking trays - about 10.
Bake for 12-15 minutes until golden and crisp. Let cool on a wire rack.