Butter a 13x9 baking dish
Spread Nutella on bottom half of a croissant. Place top half over the croissant and cut into slices. Repeat with remaining croissants, and place them in the baking dish.
In a large mixing bowl, whisk together the eggs, milk, half and half, sugar, vanilla, and salt until combined.
Pour egg mixture over the croissants.
Cover and refrigerate overnight.
Preheat oven to 350 degrees Fahrenheit.
Top with chocolate chunks, if using.
Cover loosely with foil for the first 15 minutes, then remove. Bake for an additional 20-30 minutes.
