Lightly grease a 9x13 inch-baking dish with nonstick cooking oil spray.
In a large bowl, whisk together eggs, milk, cream, sugar, vanilla, cinnamon and salt.
Halve croissants horizontally (like a sandwich). Arrange the bottom slices in the base of your dish.
Pour over half of the egg mixture. Drizzle with maple syrup.
Add the remaining croissants over the top and pour remaining egg mixture over the croissants. Cover and place in the fridge for 30 minutes or overnight.
When ready to bake, remove baking dish from the refrigerator and let stand for at least 30 minutes to get the chill out.
Preheat oven to 375°F (175°C).
Top croissants with thinly sliced butter and dust with cinnamon and vanillin sugar.
Transfer to the oven and bake for 45-50 minutes or until croissants are golden. If the tops of the croissants begin to brown too quickly, tent dish loosely with foil.
Serve croissant french toasts warm with powdered sugar, ice cream, maple syrup and fruit (if you desire).