Pour melted butter into a 9 x 13-inch baking dish. Set aside.
In a small bowl, stir together brown sugar, cinnamon and nutmeg.
In a separate bowl, whisk together vanilla extract, eggs and milk.
Arrange 6 bread slices in a single layer on the bottom of the buttered baking dish. You can cut the bread, as necessary, to fit the bottom of the dish.
Sprinkle half of the sugar mixture over top.
Add another layer of bread slices. Pour egg mixture over the bread, and gently push down on the bread to make sure that they are evenly submerged.
Sprinkle remaining sugar mixture over top.
Cover with foil and refrigerate for at least 1 hour, or overnight. The longer it sits, the better!
When ready to bake: Preheat oven to 350F/180C.
Cover dish with foil and bake for 30 minutes. Uncover and bake for an additional 20 minutes, or until the casserole is set and the top is golden brown.
Drizzle with maple syrup or dust with powdered sugar just before serving.