Lay bread slices in the dish. You will have to cut some slices in half so they all fit. Set aside.
Whisk together the milk, eggs, egg whites, 1 ½ teaspoon cinnamon, and ⅓ cup brown sugar.
Pour the mixture over the bread.
Cover {with foil} and refrigerate at least 8 hours or overnight.
Take out of the refrigerator and preheat oven to 350°F.
In a small bowl, mix the remaining ½ teaspoon cinnamon and 2 tablespoon brown sugar together. Cut the 2 tablespoon butter in with two knives or just use your fingers to combine everything. It should be crumbly and look like wet sand.
Uncover casserole; sprinkle evenly with cinnamon-sugar-butter mixture.
Put cover back on, and bake, covered, for 50 minutes in the preheated oven.
Remove cover and bake another 10 minutes until casserole is set and browned.
Remove from oven; let sit for 5 minutes before serving.
Sprinkle with powdered sugar, or serve with {a little} butter & maple syrup.