Peel and cut the sweet potato into 1-inch cubes. Place in a pot with water and bring to a boil. Cook until fork-tender, about 10 minutes. Drain, then mash until smooth (you should have about 1 cup). Set aside to cool. Alternatively, microwave or roast the sweet potato, check notes.
Melt 5 tablespoons of the butter and set aside to cool.
Preheat the oven to 400°F and place a dry 10-inch cast iron skillet inside while the oven heats.
In a medium bowl, whisk together the cornmeal, flour, granulated sugar, brown sugar, baking powder, salt, baking soda, cinnamon, and nutmeg. Set aside.
In a large bowl, whisk together buttermilk, eggs, and mashed sweet potatoes. Stir in the melted butter last, mixing until smooth.
Gradually fold the dry ingredients into the wet ingredients using a spatula, stirring until just combined. A few lumps are fine—do not overmix.
Carefully remove the hot cast iron skillet from the oven and immediately add 3 tablespoons of butter, swirling to coat. Pour the batter into the pan and smooth the top.
Return to the oven and bake for 25-30 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean (a few crumbs are okay, but no wet batter).
Let the cornbread cool in the skillet for at least 10 minutes before slicing. Serve warm with butter, honey, or maple syrup.
Notes
Alternative cooking methods:
Microwave: Pierce the sweet potato and microwave on high for 5-7 minutes, flipping halfway, until soft.
Roasting: Bake at 400°F for 45-50 minutes until fork-tender, then scoop out the flesh.
If using a 9x9 baking pan, instead of a cast iron skillet, do not preheat it in the oven. Butter the pan, then add the batter. Bake for a few extra minutes as needed.
The toothpick may have some crumbs clinging to it, but there should be no raw batter.
For extra flavor, brush the warm cornbread with melted butter or honey butter right after baking.
If you don’t want to use a cast iron skillet, use a greased 9x9 pan, but skip preheating it in the oven. Add a few extra minutes to the baking time as needed.