Butter a 13 x 9-inch baking dish with additional butter (use a little of the "plus more butter" from the ingredient list). Arrange the sourdough bread cubes (from the 14.5–16 oz loaf, cut into 1-inch slices and then 1-inch cubes) in an even layer in the dish.

In a mixing bowl with a pour spout, whisk together 3 Tbsp granulated sugar, 3/4 tsp ground cinnamon, and 1 pinch ground nutmeg.

Add all 6 large eggs to the sugar-spice mixture and whisk until well blended, scraping the bowl as needed.

Pour in 1 cup milk (not skim), 1 cup half-and-half, and 1 1/2 tsp vanilla extract. Whisk until the custard is smooth and fully combined.

Slowly and evenly pour the custard over the bread cubes, pausing and pressing down any dry pieces so they soak in the custard. If a few cubes remain dry, turn or dunk them into the custard in the dish.

Cover the baking dish tightly with plastic wrap and refrigerate overnight or for at least 10 hours.

About 30–45 minutes before baking, remove the casserole from the refrigerator. Preheat the oven to 350°F (175°C). Let the chilled casserole sit at room temperature for about 15 minutes while the oven preheats.

While the oven is preheating, make the crumble topping: in a medium bowl whisk together 1/3 cup (46 g) all-purpose flour, 1/3 cup (78 g) packed light brown sugar, 3/4 tsp ground cinnamon, and 1/8 tsp salt.

Add 1/4 cup (56 g) cold unsalted butter, diced into small cubes, to the dry mixture. Use a pastry cutter, two forks, or your fingertips to cut the butter into the flour mixture until it resembles coarse crumbs. If your oven is not yet fully preheated, place the crumble in the refrigerator until ready to use.

Sprinkle the crumble evenly over the top of the soaked bread in the baking dish.

Bake in the preheated 350°F oven until the casserole is set, the center is cooked through and no longer soggy, and the crumble is golden, about 38–45 minutes. Start checking at 38 minutes; a toothpick inserted in the center should come out mostly clean.

Remove from oven and let cool for a few minutes. Pour the 2 Tbsp melted butter evenly over the top. Dust with about 1 tsp powdered sugar if desired, cut into squares, and serve warm with real maple syrup or pancake syrup.