Prep Your Pan. Grease a 9x13" (3-quart) baking dish with a little butter or spray oil.
Prep Bread. Place dry bread cubes in the prepared baking dish.
Make The Custard Mixture. In a medium bowl, whisk together eggs, milk, syrup/sugar, melted butter, cinnamon, vanilla, and salt. (The cinnamon may look a little clumpy. Don't worry!)
Pour the egg/milk mixture over the bread in the baking dish, doing your best to evenly coat the bread. Use a spatula to *gently* pat down the bread so it's all coated in the egg mixture.
Cover & Chill. Cover the baking dish with aluminum foil and refrigerate 8-12 hours overnight in the refrigerator.
The Next Morning, Preheat the oven to 350 degrees F and let the casserole come up to room temperature while the oven preheats.
Bake. Place the baking dish in the oven and bake 30 minutes covered.
Remove the foil and bake the French toast casserole an additional 15-20 minutes or until the top is golden.
Cool 5 minutes or so, then cut into slices + top with powdered sugar or your favorite garnishes.