Dry the bread: Cut the bread into 1-inch cubes and let them sit out uncovered overnight to dry slightly. If you're short on time, spread the cubes on a baking sheet and toast at 300°F for about 10 minutes until firm but not browned.

Grease the dish: Rub the softened butter over the bottom and sides of a 9x13-inch baking dish so the casserole doesn't stick. Add the dried bread cubes in an even layer.

Whisk the custard: In a large bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, half the cinnamon, vanilla extract and salt until smooth. Whisk until there are no streaks of egg white visible.

Soak the bread: Pour the custard evenly over the bread cubes in the dish. Press down gently with a spatula so every piece absorbs some liquid.

Chill the casserole: Cover the dish tightly with plastic wrap or foil and refrigerate for at least 1 hour, or overnight for the best texture. This soaking time is what gives the casserole its soft, custardy center.

Preheat the oven: When you're ready to bake, preheat the oven to 350°F and remove the dish from the fridge while it heats up.

Make the streusel: In a small bowl, combine the brown sugar, flour and remaining cinnamon. Cut in the cold, cubed butter with a fork or your fingers until the mixture looks like coarse, pea-sized crumbs.

Top the casserole: Sprinkle the streusel mixture evenly over the soaked bread, covering as much of the surface as you can.

Bake until golden: Bake uncovered for 40 to 45 minutes, until the top is deep golden brown and the center feels set when lightly pressed. If the top browns too fast, loosely tent it with foil for the last 10 minutes.

Check the center: Insert a knife into the middle of the casserole; it should come out mostly clean with just a little moisture, not pooled liquid.

Rest before slicing: Let the casserole cool on the counter for about 10 minutes so the layers firm up for cleaner slices.

Serve warm: Cut into squares and serve with warm maple syrup drizzled over the top.