Generously butter a round dutch oven or casserole dish. Arrange bread slices in dish, slightly overlapping the slices.
In a large bowl, whisk together the eggs, eggnog, heavy cream, sugars, bourbon, vanilla, nutmeg and cinnamon. Pour the liquid evenly over the bread. You may need to rotate some of the slices to make sure they are covered in the custard mixture.
Cover and refrigerate overnight.
Preheat oven to 350°F.
Place dish in oven and bake, uncovered, for 50 - 60 minutes, or until the bread turns golden brown and puffy and the custard is set firm. In the last 10 minutes of baking, place a dome of aluminum foil over the top of the dish to protect it from burning.
Remove from the oven and cool. Garnish with powdered sugar and maple syrup if desired.