Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or nonstick spray.
In a small saucepan, melt the butter over medium heat. Stir in the brown sugar until smooth and glossy, then pour the mixture evenly into the bottom of the prepared baking dish.
Spread the bread cubes evenly over the brown sugar layer.
In a large bowl, whisk together the eggs, milk, heavy cream, brown sugar, vanilla, cinnamon, nutmeg, and salt until fully combined. Slowly pour the custard over the bread, pressing the cubes down gently so they absorb the mixture.
If time allows, cover and refrigerate for at least 30 minutes or overnight for a richer, more custardy texture.
Sprinkle the remaining brown sugar and cinnamon evenly over the top.
Bake until the casserole is puffed, golden brown, and the center is set. Let it rest for 10 minutes before serving.
Dust with powdered sugar and drizzle with warm maple syrup if desired.