In a bowl, cream the butter and both types of sugar until light and creamy. Add the egg and vanilla extract, mixing until incorporated. Then, do the same with the banana.
Add the flour, oat flakes, baking powder, and salt all at once. Knead until the dry ingredients are incorporated. Finally, knead in the chocolate chips evenly. The dough will be quite sticky, which is fine and will produce the best results.
Place the dough in the refrigerator for at least one hour to firm up.
Then, shape the dough into balls, place them on a baking sheet lined with parchment paper, and flatten them slightly.
Bake the banana bread cookies at 350°F/180˚C (conventional oven) for 17 minutes, or until the edges are golden brown.
How to store
The cookies will keep for a week in an airtight container. You can also freeze them individually for a tasty snack for up to 3 months.