Place the 1 lb of cubed brioche or challah evenly in the prepared baking dish. If using berries or orange zest, sprinkle 1 cup berries and the orange zest evenly over and between the bread cubes.

Make Custard & Assemble

Whisk custard

In a large mixing bowl, whisk together 8 large eggs until blended. Add 2 cups whole milk, 1 cup heavy cream, 1 cup granulated sugar, 2 teaspoons vanilla extract, 1 teaspoon ground cinnamon, and 1 teaspoon salt. Whisk until sugar is mostly dissolved and mixture is homogeneous (about 30–60 seconds).

Pour over bread

Pour the custard evenly over the bread cubes in the baking dish, pressing lightly on the bread with the back of a spoon to help it absorb the liquid. Allow to sit for 10–15 minutes so the bread soaks up the custard; if you prepared this the night before, cover and refrigerate for 6–12 hours.

Prepare Streusel Topping

Combine dry topping ingredients

In a small bowl, combine 3 tablespoons all-purpose flour, 3 tablespoons packed light brown sugar, 1 teaspoon ground cinnamon, and 2 tablespoons rolled oats (if using).

Cut in butter

Add 2 tablespoons cold unsalted butter pieces and use a fork or a pastry cutter to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

Top casserole

Sprinkle the streusel evenly over the soaked bread and custard, distributing it to cover the surface.

Bake & Finish

Bake

Place the dish in the preheated 350°F (175°C) oven and bake uncovered for 35–45 minutes, or until the custard is set in the center and a knife inserted near the middle comes out mostly clean with only a few moist crumbs. The top should be golden and the streusel lightly crisped.

Rest

Remove from oven and let rest for 10 minutes to firm up before serving. Dust with 2 teaspoons powdered sugar if desired.

Serve

Cut into squares and serve warm with maple syrup to taste. Leftovers can be refrigerated, covered, for up to 3 days and reheated in a 325°F oven until warmed through.