Spray a 9x13" glass baking dish with non-stick spray.
Layer pieces of bread in baking dish.
Whisk together eggs, milk, heaving whipping cream, cake mix, cinnamon, vanilla extract, 3 1/2 Tbsp sprinkles in a large bowl. Pour mixture over bread.
Cover baking dish and store in the fridge overnight or for several hours.
In a small bowl cut butter into cake mix. Store in fridge overnight.
Preheat oven to 350 degrees.
Remove french toast casserole from fridge, uncover and sprinkle with cake mix and butter mixture. Sprinkle with the rest of the sprinkles.
Bake for 1 hour until the center is set and edges are browned.
Mix together the icing mixture and pour over the top of the french toast.