Start by placing 1 inch of water in a large pot and fitting it with a steamer basket. Add in the sweet potatoes, cover, and bring to a boil. Reduce to a low boil and cook until the sweet potatoes are easily pierced with a fork, about 10 to 15 minutes.
Once cooked, transfer the sweet potatoes to a food processor and process until smooth. Measure out 1 cup for the cornbread and save any remaining puree for another use.
Preheat your oven to 375°F. While it's warming up, spray an 11×7-inch baking pan with oil to prevent sticking.
In a large bowl, whisk together the cornmeal, flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a medium bowl, mix the pureed sweet potato, granulated sugar, brown sugar, milk, sour cream, eggs, and melted butter until well combined.
Pour the wet ingredients into the bowl with the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are perfectly fine.
Pour the batter into the prepared baking pan and spread it out evenly.
Bake in the preheated oven for 35 to 45 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Let the cornbread cool in the pan for about 10 minutes before serving warm.