Preheat your oven to 350°F (175°C).
Cut each croissant into bite-sized pieces and place them evenly in the greased 9x13-inch baking dish.
In a mixing bowl, whisk together the eggs, whole milk, half-and-half, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined.
Pour the egg mixture evenly over the croissant pieces, making sure to coat all the croissant pieces thoroughly.
Press down gently on the croissants with a spatula to ensure they soak up the egg mixture.
If preparing ahead of time, cover the dish with plastic wrap and refrigerate for at least 1 hour or overnight.
When ready to bake, remove the plastic wrap and bake the casserole in the preheated oven for about 45 minutes, or until the top is golden brown and the custard is set.
Remove from the oven and let it cool slightly before serving.
Dust with powdered sugar and serve with fresh berries, if desired.