Butter a 9-inch round or 8-inch square baking dish.**
In a large bowl, whisk together the egg yolks, milk, cream, sugar, vanilla, cinnamon, and salt.
Add the croissants and chocolate. Mix until combined.
Transfer the mixture to the prepared dish, pressing the bread down into liquid if needed. Allow to sit at room temperature while you heat the oven.
Preheat oven to 375°F (190°C).
Bake 35 to 40 minutes, or until a knife inserted into the center comes out clean.
Serve warm or at room temperature.
Notes
*We are big fans of cinnamon around here and didn't find this to have an overly bold cinnamon flavor. If you like a little more cinnamon subtlety, feel free to decrease this amount.
**You can substitute any shallow 2-quart baking dish.
Store in an airtight container in the refrigerator up to 3 days.