Generously butter a 9x13-inch baking dish. Arrange bread cubes evenly in the dish.
In a large bowl, whisk together eggs, pumpkin puree, milk, heavy cream, brown sugar, vanilla, pumpkin pie spice, cinnamon, and salt until smooth and well combined.
Pour the pumpkin mixture evenly over the bread cubes, pressing down gently to ensure all pieces are soaked. Cover with plastic wrap and refrigerate for at least 2 hours or overnight.
When ready to bake, preheat oven to 350°F. Remove casserole from refrigerator and let sit at room temperature while oven heats.
Remove plastic wrap and bake uncovered for 45-50 minutes, until the top is golden brown and the center is set (a knife inserted in the center should come out mostly clean).
Let cool for 5-10 minutes before serving. Dust with powdered sugar and serve warm with maple syrup.