01 - Preheat oven to 350°F. Generously grease a 9×13-inch baking dish with butter or nonstick spray.
02 - Arrange the cubed croissant pieces evenly across the bottom of the prepared dish. Scatter the blueberries and lemon zest, if using, uniformly over the croissant layer.
03 - In a large mixing bowl, beat the softened cream cheese with an electric mixer until completely smooth and free of lumps. Add the granulated sugar and vanilla extract, blending until fully combined.
04 - Add the eggs one at a time, beating well after each addition until incorporated. Pour in the whole milk and heavy cream, whisking until the custard is silky, smooth, and easily pourable.
05 - Pour the custard mixture evenly over the croissants and blueberries. Gently press down on the croissant pieces with a spatula to submerge them and ensure even soaking throughout.
06 - Allow the assembled casserole to rest at room temperature for 10 to 15 minutes so the croissants fully absorb the custard.
07 - Bake uncovered for 35 to 40 minutes, or until the custard is firmly set in the center and the top is golden brown.
08 - Remove from the oven and let cool for 10 minutes. Dust generously with powdered sugar and serve warm.