Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or cooking spray.
In a large mixing bowl, whisk together the eggs, milk, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined.
Add the torn croissant pieces and sliced bananas to the egg mixture, gently folding until the croissants are well coated. If using, fold in the chopped walnuts.
Pour the mixture into the prepared baking dish, spreading it evenly. Drizzle the maple syrup over the top.
Cover the dish with aluminum foil and let it sit for about 30 minutes to allow the croissants to soak up the mixture.
Bake in the preheated oven for 30 minutes covered, then remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and the casserole is set.
Let it cool for a few minutes before serving. Dust with powdered sugar if desired and serve warm with extra maple syrup.