Grease a 9x13 inch baking dish or a large oval ceramic dish with butter.

Tear or cut the croissants into large, bite-sized pieces. Spread half of them into the prepared baking dish.

Scatter half of the raspberries and blackberries over the croissant pieces. Top with the remaining croissants and then the remaining berries.

In a large mixing bowl, beat the softened cream cheese and sugar until smooth. Add eggs one at a time, mixing well after each.

Whisk in the milk, heavy cream, vanilla extract, and lemon zest until fully combined and smooth.

Pour the custard mixture evenly over the croissants and berries. Gently press down on the croissants with a spatula to ensure they absorb the liquid.

Let the mixture sit for at least 20 minutes (or refrigerate overnight for best results) to allow the bread to soak up the custard.

Preheat oven to 350°F (175°C).

Bake for 35-45 minutes, or until the custard is set and the tops of the croissants are golden brown and crisp. Cover loosely with foil if browning too quickly.

Remove from oven and let cool for 10 minutes. Dust generously with powdered sugar before serving.