Babka French Toast Bake

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1. 1
   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.
2. 2
   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.
3. 3
   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.
4. 4
   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.
5. 5
   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.