Preheat the oven to 375°F.
Butter an 8-inch square (or other shallow 2-quart) baking dish.
butter
In a large bowl, whisk together the egg yolks, milk, cream, sugar, vanilla, salt, and nutmeg until smooth.
6 large egg yolks, 2 cups whole milk, 1 cup heavy cream, 1 cup granulated sugar , 1 tsp vanilla extract , 1/2 tsp kosher salt , 1/2 tsp ground nutmeg
Add the toasted croissant pieces and chopped chocolate to the bowl. Stir to combine.
6 croissants (1 lb) cut into 1" pieces (it's okay if they are day old), toasted in the oven until they feel dry and hard, but are still very pale looking*, 4 oz bittersweet chocolate (70% cacao) chopped into chunks
Transfer the mixture to the prepared baking dish.
Bake for 40-50 minutes, or until a knife inserted in the center comes out clean. Alternatively, check that an instant-read thermometer reads at least 160°F and the top is golden brown.
Serve warm or at room temperature.