Instructions:
- Preheat oven to 350°F (175°C). Grease an 8x8 inch baking pan and line thoroughly with parchment paper, ensuring a generous overhang on two sides to use as a lift sling.
- In a large bowl, thoroughly mash the ripe bananas until mostly smooth. Stir in the peanut butter and vanilla extract (if using) until well combined into a homogenous liquid.
- Add the rolled oats, brown sugar, and cinnamon (if using) to the wet mixture. Use a spatula to fold everything together until just combined. Do not overmix.
- Transfer the mixture into the prepared pan. Use the bottom of a measuring cup or your knuckles to press the mixture down firmly and evenly across the base, packing the corners tightly.
- If using, sprinkle the chocolate chips evenly over the top surface. Bake for 25 to 30 minutes, or until the edges are set and lightly golden brown.
- Remove from the oven and let the bars cool completely in the pan on a wire rack. For the cleanest cuts, chill them in the refrigerator for at least 1 hour after they have cooled down.
- Use the parchment paper sling to lift the entire slab out of the pan. Slice into 12 or 16 squares and serve.