For the French toast: Preheat the oven to 375 degrees F. Generously grease a 9-by-9-inch baking dish with the butter.
Place the bread cubes in the prepared baking dish.
Whisk together the eggs, half-and-half, sugar, vanilla and salt in a large bowl. Pour the egg mixture over the bread. Gently push the bread down to absorb the egg mixture. Dollop the chocolate-hazelnut spread over the bread and mix it in until evenly distributed.
For the topping: Combine the butter, oats, pecans and brown sugar in a food processor and pulse until it resembles a coarse meal.
To finish the dish: Top the bread cubes with the oatmeal mixture. Wrap tightly with foil and refrigerate at least 6 hours or overnight.
Bake, uncovered, until the edges are bubbling and the top is golden brown, 45 to 50 minutes. Let cool for 5 minutes. Serve with maple syrup.