Grease a 9x13-inch baking dish with some of the melted butter. Keep the rest for later. Preheat your oven to 350°F (175°C) if baking right away.
Layer half of the bread cubes in the dish. Warm the Nutella just enough so it loosens (a quick 15–20 seconds in the microwave does the trick).
Dollop half the Nutella over the bread in little spoonfuls. Scatter the remaining bread on top, then add the rest of the Nutella in more dollops. Those pockets of chocolate-hazelnut are what make each bite fun.
In a large bowl, whisk the eggs, milk, cream, sugar, vanilla, cinnamon, and salt. If using orange zest, whisk it in too. The custard should smell like dessert, but not over the top.
Pour the custard slowly and evenly over the bread. Press down gently with the back of a spoon so the top pieces soak up some liquid. Drizzle any remaining melted butter on top.
Let the casserole rest for at least 30 minutes at room temperature so the bread hydrates. If you’ve got time, cover and refrigerate overnight. The overnight rest gives you a softer, more custardy center and makes the morning easier.
When ready to bake, uncover and place the dish on the middle rack. Bake for 35 to 45 minutes. The edges should be puffed and golden, and the center should be set but still a little soft. If it’s browning fast, tent loosely with foil.
Rest for 10 minutes before serving. This quick pause lets the custard settle. Sprinkle with powdered sugar, top with hazelnuts if using, and add fresh berries or bananas. Maple syrup on the table is never a bad idea.