Ingredients
Method
Spread a thin layer of ⅔ cup Nutella or other chocolate hazelnut spread on one side of each slice of one 1-lb. loaf challah or white Pullman bread, sliced ½" thick, then cut in half on a diagonal. Set aside.
Whisk 8 large eggs, 3 cups heavy cream, 1 Tbsp. vanilla extract, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ½ tsp. ground cinnamon, and ⅔ cup (packed; 133 g) dark brown sugar in a large bowl until smooth. Pour half of custard into a 13x9" baking dish. Shingle reserved bread, Nutella side up, in dish, overlapping as needed to create a zigzag-like pattern. Pour remaining custard over to saturate bread. Cover tightly with plastic wrap and chill at least 1 hour.
Place a rack in middle of oven and preheat to 375°. Remove dish from fridge and with plastic wrap still on, gently press down on bread to settle further into dish and create a more flat, shingled appearance. Uncover and bake until golden brown, 35–40 minutes.
Meanwhile, heat remaining ⅓ cup (packed; 67 g) dark brown sugar and 3 Tbsp. water in a small saucepan over high, stirring often, just until sugar is dissolved and syrup is hot, 2–3 minutes.
Immediately brush all of syrup evenly over casserole. Bake until top is shiny and candy-like and bread is a shade darker, 8–10 minutes. Let cool slightly before serving. Do Ahead: Unbaked casserole can be assembled 1 day ahead. Keep chilled.