Place croissant pieces into a greased 1 quart Pyrex (linked below). Fit as many as you can without smushing down.
In a medium bowl, whisk together egg, sugar, heavy cream, vanilla, cinnamon or pumpkin pie spice, and salt.
Pour cream mixture over croissants, making sure everyone gets wet.
Let sit for 15 minutes for maximum absorption, tampon-style. Gently toss pieces halfway through soak time. Again, no smushing allowed!
While croissants are swimming, preheat oven to 350°F.
when ready to bake, cover Pyrex with tinfoil and poke several holes through the top with a toothpick.
Bake for 45 minutes. Then, remove tinfoil to brown the top for 15 minutes. Bread pudding will have risen when done, and center should have minimal jiggle. As it cools, it will shrink back down.
Let cool a bit before serving, then top with a fat scoop of ice cream and a drizzle of some warm caramel sauce. Fuuuuuuuck.