Butter a 9-by-13-inch baking dish. Split croissants horizontally, keeping a hinge along one edge if possible. Mix ricotta with 1 tablespoon sugar and 1 teaspoon orange zest, then fold in chopped chocolate.
Spread one-quarter of the filling inside each croissant and close gently. Arrange the four filled sandwiches in the dish in a single snug layer.
Whisk eggs, milk, cream, vanilla, salt, the remaining 2 tablespoons sugar and remaining teaspoon zest. Pour slowly around and over the sandwiches, spooning a little custard onto the cut edges.
Let stand for 20 minutes while heating oven to 350°F, spooning pooled custard over the lower pastry once halfway through.
Bake for 35 to 40 minutes until the custard between the sandwiches reaches 160°F and is set with a slight wobble. Tent loosely with foil after about 20 minutes if the tops are already golden.
Rest for 10 minutes, then lift one filled croissant with its surrounding custard onto each plate using a broad spatula.