Leftover croissants may seem like a stretch, but there's an easy work around to ensure you can enjoy this savory bread pudding. Some bakeries offer 2-for-1 deals on pastries after 4:00 p.m., so stock up on them then.
In a deep bowl, beat together the eggs, milk and salt if using with a fork. Add half of the prosciutto and provolone, and all of the croissant pieces. Use a rubber spatula to stir until well mixed. Pour into the prepared pan. Press the croissants down with the spatula to help them absorb the milk mixture. Sprinkle the remaining cheese and prosciutto on top. Cover with foil, and refrigerate at least 2 hours or up to overnight.
Preheat the oven to 350 degrees F. Bake for 20 minutes. Remove the foil, and bake until puffed and the cheese is golden, 30 minutes more. Serve hot.
The bread pudding will fall, like a souffle, a few minutes after you take it from the oven, so bring it to the table promptly.