1. Generously grease a 9 X 13 deep-dish casserole dish with 1 tablespoon of butter. Place a layer of bread slices alternating direction of bottom crusts and top crusts to outside of pan. Add a layer of bananas and chocolate chips. Repeat 2 more times ending with layer of bread.
2. In a large bowl whisk eggs, milk, butter and vanilla together. Slowly pour over bread layers making sure to coat evenly.
3. In a small bowl combine oatmeal, butter and brown sugar and mix into clumps with your fingers. Top casserole with oatmeal clumps. Wrap tightly with plastic wrap and refrigerate overnight.
4. To bake, preheat oven to 350 degrees. Remove casserole from refrigerator and bring to room temperature, about a half an hour. Bake uncovered for 35-45 minutes or until edges are bubbling and top is golden. Remove and let set for 5 minutes and serve.