Preheat oven to 350°F. Spray a 9×9-inch baking dish with cooking spray.
Break croissants into bite-sized pieces and spread evenly in prepared baking dish. Scatter strawberries, blueberries, and raspberries over croissant pieces.
In a medium bowl, beat cream cheese with electric mixer until fluffy and smooth.
Gradually add sugar, milk, eggs, and vanilla extract to cream cheese, mixing until well combined and silky.
Pour cream cheese mixture evenly over croissants and berries. Gently press down with spatula to ensure all pieces are soaked.
Cover dish and refrigerate for at least 15 minutes, or up to 14 hours overnight.
Remove cover and bake for 35-40 minutes until golden brown and center is set but slightly jiggly.
Let cool for 10 minutes before serving. Dust with powdered sugar and drizzle with maple syrup if desired.