Scatter bread cubes evenly in a greased 9x13-inch baking dish.
Whisk together eggs, milk, heavy cream, vanilla, and brown sugar in a medium bowl until smooth.
Pour the egg mixture evenly over the bread cubes. Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight.
For the topping, mix flour, 1/2 cup packed light brown sugar, cinnamon, and salt in a medium bowl. Cut the butter into this mixture until crumbly. Refrigerate for at least 4 hours, or overnight.
Preheat the oven to 350°F. Remove the plastic wrap from the bread mixture and sprinkle the topping evenly over the dish. Bake uncovered for 45-60 minutes, until the top is golden brown and the center is set.
Serve with a drizzle of maple syrup or a dusting of powdered sugar for added sweetness.