Preheat the oven to 350°F. Spray a 9-inch square baking dish with nonstick cooking spray.

Whisk together the mashed bananas, applesauce, honey, stevia (if using), egg, and vanilla in a medium bowl until combined. Add the oat flour, cocoa powder, baking powder, baking soda, and salt. Stir with a rubber spatula until the mixture becomes a smooth batter. Fold in the chocolate chips (if using).

Pour batter into the prepared baking pan.

Bake the brownies on middle rack of oven until a toothpick inserted in the center comes out clean, 25-27 minutes.

Check to see that brownies are done. Remove from oven or add time as needed.

Allow brownies to cool to room temperature, then transfer to refrigerator to cool for at least 2 hours.

Remove brownies from refrigerator, cut into squares, and serve.