First, preheat the oven to 350 degrees F and find a shallow casserole dish. Gently tear the croissants into roughly 1-inch pieces and stuff them into the bottom of the casserole dish, making sure they're packed tightly in a single layer. (Note that for scaling this recipe, you will have to adjust the size of your casserole dish.)
Next, make the french toast custard. In a large bowl, whisk together the eggs and milk. Then add the vanilla extract, light brown sugar, cinnamon, nutmeg, and allspice, and whisk thoroughly to combine.
Finally, pour the custard evenly over the croissant pieces in the casserole dish, making sure the croissants are covered completely in liquid.
Bake for 40 to 60 minutes, until slightly puffed and golden brown and a toothpick inserted in the center comes away clean. Let cool at least 5 minutes.
Optionally dust with powdered sugar, then serve immediately with maple syrup.