Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper or the exposed sides of the pan.

In a large mixing bowl, whisk together the rolled oats, all-purpose flour, light brown sugar, baking powder, ground cinnamon, and salt. Make sure there are no lumps of brown sugar.

In a separate medium mixing bowl, mash the ripe bananas thoroughly with a fork or potato masher until smooth. Add the eggs, melted butter (or coconut oil), vanilla extract, and milk to the mashed bananas. Whisk until well combined.

Pour the wet ingredient mixture into the dry ingredient mixture. Using a rubber spatula, stir until just combined. Be careful not to overmix. If using, gently fold in the chocolate chips, chopped nuts, or dried fruit.

Transfer the batter to the prepared 9x13 inch baking pan. Use the spatula to spread it evenly into a flat layer.

Bake for 25-30 minutes, or until the edges are lightly golden brown and a toothpick inserted into the center comes out clean. The bars should feel firm to the touch.

Remove the pan from the oven and let the bars cool completely in the pan on a wire rack before attempting to cut them. This is crucial for them to set properly and prevent crumbling.

Once fully cooled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Place on a cutting board and cut into 16 equal squares or bars.