You want an eggy, finely textured bread for this, such as challah or brioche. I've kept it on the less sweet side, so you can liberally douse it with maple or your syrup of choice in the morning.
Makes 4 servings
For The French Toast
8 slices of challah, 3/4-inch thick (or enough to snugly fill a standard 9-by-13-inch dish)
6 large eggs
2 cups milk
1/4 cup light brown sugar
1 teaspoon vanilla
1 teaspoon cinnamon
1 teaspoon salt
For The Topping
1 cup whole hazelnuts (shelled but no need to skin them)
1/4 cup sugar
4 tablespoons cold unsalted butter, cut into a few pieces
1/4 cup cream
The Night Before
Set the slices of bread snugly in a 9-by-13-inch glass baking dish. Beat or blend together the eggs, milk, brown sugar, vanilla, cinnamon and salt.
Pour the liquid mixture over the bread and let it soak for about 15 minutes. Then flip the bread over so the other side can soak. Press a sheet of wax paper right against the damp bread and slide the whole thing into the fridge.
For the topping, in a food processor pulse the hazelnuts and sugar until the hazelnuts are little bigger than rice grains. Add the butter and cream and pulse until you have a gritty, cohesive mixture. Scrape this mixture into a bowl, cover with plastic wrap, and store in the fridge.
The Next Morning
Preheat the oven to 350 degrees. Take the soaked French toast out of the fridge and discard the wax paper. Strew the topping all over it and bake in the oven for 40 to 45 minutes. When the liquid has been completely absorbed and the bread is puffed and golden, it's done. Serve immediately with as much maple syrup as you like.