Peel sweet potatoes and cut into chunks. Add to a pot and cover with water. Bring to a boil and cook for about 10 minutes, until easily pierced with a fork. Drain and mash until smooth. Set aside.
Preheat oven to 375°F and lightly grease a 9x9-inch square pan or cast iron skillet.
In a large bowl, mix together butter and sugar. Beat in eggs, one at a time, then mix in mashed sweet potato and milk until smooth.
In another bowl, stir together cornmeal, flour, baking powder, baking soda, salt, and pepper. Pour into wet ingredients and stir just until incorporated and no dry streaks remain.
Pour into prepared pan and bake for 25-30 minutes, until golden and edges are pulling away from the sides. Let cool slightly before serving.
Notes
If using this recipe for muffins, reduce baking time to 15-18 minutes. A toothpick inserted into the center should come out clean.