Carefully slice each croissant in half, lengthwise; set aside.

Place flour into a shallow dish, then very gradually whisk in the eggnog--the slower, the less lumpy it will be. Whisk in the egg yolks, egg, vanilla, spices, and salt until smooth.*

Working with 2-4 halves at a time, soak croissants in the custard until saturated but not falling apart, about 30-45 seconds per side.

While the croissants are soaking, heat a nonstick skillet over medium heat and coat with nonstick cooking spray or oil. Lift soaked croissants out of the custard, allowing the excess to drip off, and transfer to the heated skillet (cut-side down first) to cook until golden brown, about 3-4 minutes. Flip, then immediately top croissant halves with dollops of Nutella, about 1-2 Tablespoons per half. Continue to cook until the second side is golden brown, about 3-4 minutes longer.

Once browned, use a spatula to carefully flip one half onto the other half, forming a sandwich. Transfer Nutella sandwiches to a plate, dust with powdered sugar, and drizzle with maple syrup, if desired.