Spray 10 inch cast iron skillet with cooking spray and place in oven, preheating to 425°F.
In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, salt and pumpkin pie spice.
In a medium bowl, whisk together the buttermilk, mashed sweet potatoes, melted butter and eggs. Stir the wet ingredients into the dry ingredients until just combined.
Take the hot skillet out of the oven and pour the cornbread mixture in, making sure it's spread evenly. Bake for 20 to 25 minutes, or until the top is puffy and golden brown. There should be some cracks around the edge as well.
Notes
Use baking powder that is less than a year old since it has been opened. If you use older baking powder, it won’t be powerful enough to help leaven the cornbread. You can test it by stirring half a teaspoon of baking powder into a cup of hot water. It will immediately start to fizz and release carbon dioxide gas if it’s still fresh enough to use.
Just mash the potatoes. Do not puree it as you want small pieces of sweet potato throughout, which also helps the cornbread hold up.
Do not overmix the batter. You want to fold just until everything is combined. If you overmix the bread will be tough.