Cut or tear the croissants into large bite-sized pieces and place them evenly in a greased 9x13-inch baking dish. If they're fresh, toast them in a low oven at 300°F (150°C) for 5 to 10 minutes to dry out slightly.

In a medium bowl, use a hand mixer or spoon to beat together the softened cream cheese, 1/3 cup of sugar, and 1 tsp of vanilla extract until the mixture is smooth and creamy with no lumps remaining.

Drop spoonfuls of the cream cheese mixture evenly over the croissants in the baking dish. Use a butter knife to swirl the cream cheese slightly into the croissants, then scatter the blueberries across the top.

In a separate large bowl, whisk together 4 large eggs, 1 cup of whole milk, 1/2 cup of heavy cream, 1/3 cup of sugar, 1 tsp of vanilla extract, cinnamon (if using), and a pinch of salt until fully combined and smooth.

Slowly pour the custard over the entire croissant mixture, ensuring all pieces are moistened. Use a spatula or clean hands to gently press the croissants down so they begin to absorb the custard mixture.

Cover the dish with foil or plastic wrap and refrigerate for at least 30 minutes or overnight for best results. When ready to bake, preheat the oven to 350°F (175°C).

Remove the cover and bake the casserole for 40 to 45 minutes or until the top is golden brown and the center is set. Let it rest for 10 minutes before serving.