Grease a 9x13-inch baking dish with butter. Spread bread cubes evenly in the dish.
In a large bowl, whisk together eggs, milk, cream, sugar, vanilla, cinnamon, nutmeg, and salt until smooth and well combined.
Pour the egg mixture evenly over the bread cubes, pressing down gently to ensure all pieces are soaked. If using blueberries, sprinkle them over the top.
Drizzle melted butter over the entire dish. Cover with plastic wrap and refrigerate for at least 30 minutes, or overnight for best results.
When ready to bake, preheat oven to 350°F. Remove plastic wrap and let the dish sit at room temperature while the oven heats up.
Bake uncovered for 45-50 minutes, until the top is golden brown and the center is set (it should jiggle just slightly when you shake the pan).
Let cool for 10 minutes before serving. Dust with powdered sugar and drizzle with warm maple syrup. This is absolutely delicious served with a dollop of whipped cream or a scoop of vanilla yogurt!