In a large mixing bowl, whisk together eggs one at a time until pale yellow and thick. Slowly add in buttermilk, heavy cream, vanilla, and nutmeg. Mix well.
Carefully dip each slice of bread in the egg mixture, coating both sides. Place in a buttered roaster dish, and continue this step until the baking dish is full, making sure the slices slightly overlap one another. Cover with plastic wrap and chill in the refrigerator for at least 2 hours (or overnight).
While the bread is chilling, prepare the cinnamon sugar topping. In a small bowl, mix together flour, brown sugar, and cinnamon. Slowly add in cubed butter chunks and mix with a fork until the mixture is crumbly. Cover with plastic wrap and chill in refrigerator until ready to bake.
Preheat oven to 350°F. Remove chilled french toast from refrigerator, completely covering the top with half the cinnamon sugar mixture. Place uncovered in the oven and bake for 20-25 minutes. Halfway through, flip the french toast on the other side and cover with remaining half of cinnamon sugar mixture. Bake for another 20-25 minutes until it is crisp and golden.
While french toast is baking, prepare the orange maple syrup. In a large saucepan, melt together butter, brown sugar, corn syrup, orange juice, and zest on medium heat. Continually stir until it becomes a smooth, thick consistency and turns a caramel color, about 10 minutes.
When the french toast is done, serve immediately while warm, and top with a hefty serving of the orange maple syrup. Enjoy!
You may substitute milk for buttermilk if you do not have buttermilk on hand.