Spray a 9x13 inch baking dish with cooking spray, place the bread cubes in the pan and set aside.
In a large bowl, whisk together the chocolate milk, eggs, cinnamon, and vanilla extract. Pour the milk mixture over the bread cubes. Push down on the bread slightly if necessary to make sure each piece is coated. Cover and chill for at least 2 hours, up to overnight.
Preheat oven to 350 degrees. Allow the casserole to sit at room temperature while the oven is preheating. Bake for 20- 30 minutes, until casserole is cooked through.
Remove the casserole from the oven. Top with the chocolate chips, and marshmallows. Broil for 2-3 minutes, until chocolate is starting to melt and the marshmallows are golden brown on top.
Top with crushed graham crackers, and serve immediately.
Notes
NOTE: This french toast must chill for at least two hours and up to overnight before baking.