To start, preheat your oven to 350 degrees F. Then, generously spray a 9x13 baking dish with cooking spray. Mash the bananas until they're smooth, then add in the oats, chia seeds, cocoa powder, vanilla extract, milk, and egg whites. Give it all a good stir, then fold in the chocolate chips, saving a few to sprinkle on top. Pop it in the oven and bake for 25-30 minutes, or until the liquid is absorbed. Once it's done, take it out, serve it up, and get ready to enjoy a delicious and nutritious treat!