Spray a 9x13-inch pan (or similar casserole dish) with nonstick spray. Sprinkle the bread cubes in the prepared pan.
In a large bowl, whisk together the eggs, milk, sugar, vanilla and salt. Pour the egg mixture over the bread cubes. Cover with plastic wrap, pressing the plastic down against the bread cubes to submerge them in the egg mixture. Refrigerate for at least 4 hours or overnight.
Remove the pan from the refrigerator 30 minutes prior to baking to let it warm up closer to room temperature. Preheat the oven to 350 degrees F.
Remove the plastic wrap and discard. Sprinkle the butter pieces evenly over the casserole. Combine the sugar and cinnamon and sprinkle that evenly over the top of the casserole.
Cover the pan with foil and bake for 45 minutes. Remove the foil and bake for an additional 15 minutes. You'll see the casserole puff up considerably- it will settle down once you remove it from the oven and it begins to cool. Serve warm slices with warm maple syrup.
Notes
Maple syrup is not included in the nutritional information.
FREEZE AHEAD TIPS: If you choose to freeze French Toast Casserole before baking, prepare it until you put the egg custard over the bread cubes. Then place plastic wrap over and press down to adhere to the soaked bread. Wrap in plastic wrap again. Then wrap with foil and freeze (up to 1 month) before ready to make. When ready to make, let the frozen French Toast Casserole defrost in your refrigerator for 24 hours. Add the topping. Then proceed with the regular baking instructions.If you want to freeze French Toast Casserole after it's already baked (or if you want to freeze leftovers), you can do so for a month or two. Wrap the cooled French Toast Casserole well with plastic wrap, cover with foil and freeze. To serve, let it defrost in your refrigerator for 24 hours. Then remove the plastic wrap, cover with foil and heat in the oven for about 20 minutes at 350 degrees F.