Preheat oven to 400 degrees. Place a cast iron skillet in the oven while it is preheating.
With a fork, poke holes into the sweet potato on all sides, a total of 10 times. Microwave on a plate on high for 5 minutes, flipping halfway through cooking. Let the sweet potato sit for 3 minutes before cutting it in half. Remove the skin. Mash the cooked potato with a fork and set aside.
In a large bowl, mix sugar, flour, cornmeal, salt, baking powder, cinnamon, and nutmeg with a large whisk, making sure all the dry ingredients are thoroughly mixed together. Set aside.
In a separate bowl, whisk together eggs, milk,mayonnaise, vanilla, and the cooked sweet potato.
Pour the wet ingredients in with the dry ingredients. Gently mix the ingredients with a wooden spoon or plastic spatula. Be sure to scrape the sides of the bowl so all of the dry ingredients have been mixed in.
Once the oven is set to the correct temperature, remove the cast iron from the oven. Spread the melted butter into the cast iron, then pour in the cornbread mixture. Spread the cornbread mixture evenly in the pan with a spoon or spatula.
Bake for 25-28 minutes, or until a knife or toothpick comes out clean when inserted into the center of the cornbread.
Let the cornbread rest for 5 minutes before serving. Cut and serve with maple bourbon butter!