In a large bowl, whisk together the eggs, eggnog, heavy cream, granulated sugar, vanilla, nutmeg, and bourbon, if using. The mixture should be smooth and well combined.
Arrange the bread slices in a single layer in a 9x13-inch (23x33 cm) baking dish or other shallow dish. Pour the egg mixture evenly over the bread, ensuring each slice is saturated. Cover the dish with plastic wrap and refrigerate for at least 4 hours, or preferably overnight.
When ready to bake, preheat your oven to 375°F (190°C).
In a small saucepan over medium heat, melt the 6 tablespoons of unsalted butter. Whisk in the brown sugar and maple syrup until smooth. Pour this mixture into the bottom of a clean 9x13-inch (23x33 cm) baking dish, spreading it into an even layer.
Carefully arrange the soaked bread slices on top of the brown sugar mixture in the baking dish. Discard any unabsorbed egg mixture.
Bake for 35 to 40 minutes, or until the top is golden brown and the custard is set. The center of the dish should register at least 160°F (71°C) on an instant-read thermometer.
Let the French toast cool in the pan for 5 to 10 minutes before serving. To serve, use a spatula to lift out individual slices, turning them over to show the caramelized bottom if you like. Serve warm with powdered sugar or extra maple syrup.