Preheat oven to 350°; line two baking sheets with parchment paper or use a silpat baking sheet.

In a medium bowl, whisk together flour, baking powder, baking soda, salt and cinnamon. In a large bowl, with a mixer, cream together the butter and the sugars on medium speed. Add the egg, followed by the mashed banana and vanilla extract. Working by hand, stir in the flour mixture and the oats until just combined and no streaks of flour remain; stir in the chocolate chips.

Drop heaping tablespoonfuls of the dough onto prepared baking sheets. Bake for about 10-12 minutes, or until cookies become light brown at the edges. Let cool on baking sheet for 3 or 4 minutes, then transfer to a wire rack to cool completely.