Heat oven to 375° F. Butter an 8-inch square or other shallow 2-quart baking dish.
In a large bowl, whisk together the egg yolks, milk, cream, sugar, vanilla, salt, and nutmeg. Add the croissants and chocolate and mix to combine.
Transfer the mixture to the prepared baking dish and bake until set and a knife inserted in the center comes out clean, 30 to 40 minutes. Serve warm or at room temperature.
Make-Ahead Tip: Cut the croissants and chop the chocolate, then store them at room temperature in separate resealable plastic bags for up to 1 day.