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1
Heat the oven to 200 degrees F and arrange a rack in the middle.
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2
Place a baking sheet in the oven.
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3
Insert the blade of a paring knife through the top crust of a challah slice, forming a pocket in the center of the slice (take care not to pierce the bottom of the slice).
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4
Spread a scant 1 1/2 tablespoons of Nutella in the pocket, distributing it as evenly as you can around the sides of the pocket; set aside.
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5
Repeat with the remaining slices.
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6
Place the eggs, milk, vanilla, orange zest, and salt in a shallow dish (a 13-by-9-inch baking dish works well) and whisk until the eggs are broken up and evenly incorporated and the sugar has dissolved.
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7
Melt the butter in a large cast iron or nonstick frying pan over medium heat until foaming, about 2 minutes.
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8
Dip a few slices of the bread in the milk mixture, turn to coat, and let the excess drip off.
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9
Lay the soaked bread in a single layer in the pan and cook until browned on the bottom, about 2 to 3 minutes.
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10
Flip and cook the second side until browned, about 2 minutes.
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11
Place the cooked French toast on the heated baking sheet.
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12
Repeat with the remaining bread slices.
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13
Serve sprinkled with the hazelnuts and dusted with powdered sugar, if using.
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14
Pass maple syrup at the table.
