Preheat the oven to 300°F/150°C. Line a sheet pan with parchment paper or silicone mat and flatly layer cut croissants. Bake for 15-20 minutes or until the pieces are very dry.
Meanwhile, in a large mixing bowl whisk together cream, milk, eggs, vanilla, brown sugar, granulated sugar, salt and orange zest until well combined.
Place the toasted croissants in a greased 9 x 13-inch baking dish. Pour the custard mixture over the croissants and lightly press down to ensure most of them are submerged in the liquid.
Cover with aluminum foil and refrigerate for one hour.
Preheat the oven to 350°F/177°C.
Bake the croissant bread pudding uncovered for 45-50 minutes or until custard is set and the top is golden.
Allow to cool slightly.