Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Layer the Croissants and Berries: Place the croissant pieces in the prepared baking dish. Scatter the mixed berries evenly over the croissants.
Prepare the Egg Mixture: In a medium bowl, whisk together the eggs, milk, granulated sugar, vanilla extract, cinnamon, and salt until well combined.
Combine with Croissants: Pour the egg mixture evenly over the croissants and berries, pressing down gently to ensure the croissants absorb the mixture.
Bake: Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the egg is set.
Cool and Serve: Allow the croissant bake to cool for a few minutes before slicing. Dust with powdered sugar if desired and serve warm with maple syrup.