Grease a 9x9" square baking dish.
Cut each of the French toast sticks into 3 pieces and place them all in prepared pan.
In a large bowl, whisk together the eggs, milk, vanilla, sugar and ½ teaspoon of the cinnamon; pour over the French toast pieces.
Sprinkle the remaining ⅛ teaspoon cinnamon and the brown sugar evenly over the casserole.
Cover and refrigerate overnight.
When ready to bake, preheat oven to 350°F.
Bake the casserole, uncovered, for 35-40 minutes, or until the center is almost set; remove from the oven and let rest for 5 minutes before serving.
Sprinkle with powdered sugar, if desired, and serve warm with maple syrup.