Spray a 9x13-inch baking dish with nonstick cooking spray.
Add the bread cubes to the dish in an even layer. Set aside.
In a large bowl, whisk together the eggs, milk, heavy cream, brown sugar, vanilla, cinnamon, and salt until smooth and fully combined.
Pour the egg mixture evenly over the bread cubes, gently pressing down to ensure all of the bread gets soaked. Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or up to overnight.
In a medium bowl, whisk together the flour, brown sugar, cinnamon, and kosher salt for the topping. Add the melted butter and mix until crumbly and well combined. Cover the topping mixture and refrigerate until ready to bake.
When ready to bake, preheat the oven to 350°F (175°C).
Remove the casserole from the refrigerator. Sprinkle the prepared topping evenly over the soaked bread cubes.
Bake the casserole uncovered for 45-55 minutes, or until the top is golden brown and the center is set.
Let the casserole rest for 10 minutes before serving.
Dust with confectioners' sugar and drizzle with maple syrup.