Bring sweet potatoes, peeled and cut into chunks (about 2 or 3 medium potatoes) to a boil, covered with water, in a medium saucepan. Reduce the heat to low and simmer for about 20 minutes, until tender.
Drain thoroughly and press through a strainer into a bowl and set aside to cool slightly.
Preheat oven to 425 degrees.
Sift together the cornmeal, flour, baking powder, salt, baking soda, pepper, and nutmeg.
Add the egg and buttermilk to the sweet potato puree and mix until well blended.
In a 8 or 9 - inch ovenproof skillet, heat the butter until just sizzling - about 2 minutes. Swirl the butter in the pan to coat it well. Pour any excess butter into the sweet potato mixture.
Quickly add the cornmeal mixture to the sweet potato mixture and stir until blended and pour the batter into the skillet.
Bake for 20 or 30 minutes, until lightly golden and the corn bread has pulled away from the sides of the pan. Remove from the oven and cut into wedges.