Spray 9-inch x 13-inch baking dish with non-stick cooking spray. Set aside.
In a small saucepan bring brown sugar, butter, and corn syrup to a boil over medium heat. When the sugar is dissolved pour into the prepared pan.
Place six slices of bread on top of the caramel. You might need to squish the slices together to make them all fit.
In a small bowl mix the cinnamon and sugar together. Sprinkle half of the mixture over bread in the pan. Top with remaining six slices of bread.
In a large bowl whisk together eggs, milk, and vanilla. Stir in remaining half of the cinnamon-sugar mixture and stir until combined. Pour over bread. Cover and refrigerate overnight.
Take french toast casserole out of the fridge and let stand at room temperature for 30 minutes.
Preheat oven to 350 degrees F. Uncover dish and bake for 35-40 minutes until golden. Remove from oven. Use a spatula to flip the bread slices over onto a plate so the caramel side is up. Top with powdered sugar, if desired, and serve immediately.