Butter a 9" x 13" casserole dish with the softened butter or lightly coat with cooking spray.
Dice the bread into 1" cubes and then spread the cubes onto a baking sheet in a single layer. Transfer the bread cubes to the oven to dry out. The cubes are finished when they are firm and dry, but not toasted.
Remove the bread from the oven and spread the cubes into the prepared casserole dish.
While the bread is drying out, prepare the custard. In a large mixing bowl, combine the eggs, milk, cream, maple syrup, vanilla, cinnamon, and nutmeg. Whisk to thoroughly combine.
Pour the custard over the bread cubes. Using a spatula or a wooden spoon, gently press down on the bread to ensure each one gets coated with the custard.
Cover the casserole dish with foil and transfer to the fridge. Chill for at least 15 minutes, and up to overnight.
When you are ready to bake, preheat the oven to 350*.
Transfer the foil covered casserole dish to the oven and bake for 30 minutes. Remove the foil and bake for an additional 15 minutes until the bread pudding is golden brown on top and puffed. Remove from the oven and allow to rest for 5 minutes; it will deflate.
Dust with powdered sugar, and serve warm, drizzled with additional maple syrup. The cooked casserole will keep in the fridge, covered, for an additional 2 days.