01 - Preheat oven to 350°F and generously grease a 9x13-inch baking dish with butter or cooking spray.
02 - Arrange half the sliced croissants in the bottom of the prepared dish. Scatter half the blueberries evenly over the croissants.
03 - In a medium bowl, beat cream cheese, 1/2 cup sugar, and vanilla extract until completely smooth and creamy, about 2 minutes.
04 - Distribute the cream cheese mixture in dollops over the croissant and blueberry layer. Top with remaining croissants and blueberries.
05 - In a separate bowl, whisk together eggs, whole milk, heavy cream, and 1/3 cup sugar until fully incorporated and smooth.
06 - Pour custard evenly over the casserole. Drizzle melted butter on top. Gently press croissants down to ensure even absorption. Let stand for 10 minutes.
07 - Bake uncovered for 35-40 minutes until golden brown and set in the center. If top browns too quickly, tent loosely with foil during last 10 minutes.
08 - Let casserole cool for 10 minutes before serving. Dust generously with powdered sugar just before serving. Best enjoyed warm.