Preheat the oven to 350° and spray a 9x13 baking dish with cooking spray and set aside.
In a large bowl whisk together the milk, heavy cream, eggs, sugar, vanilla extract, salt, and cinnamon. Add the croissant cubes and mix together until the croissants are fully coated in the cream mixture. Let it sit for 25-30 minutes, stirring about 15 minutes in.
Once the croissants have soaked pour the mixture into the prepared pan. Bake for 45-50 minutes, until the custard, is set and the croissants are starting to brown. While the bread pudding is baking prepare the vanilla sauce.
In a medium saucepan combine the butter, sugar, and cream. Whisk constantly over medium heat until the sauce thickens and coats the back of a spoon. Remove from the heat and add the vanilla.
Allow the bread pudding to sit for at least 20 minutes before serving. Serve with warm vanilla sauce.