Place a 10-inch cast iron skillet on oven's center rack. Turn oven to 400 °F (204 °C), allowing the pan to preheat.
Melt butter in a small saucepan over medium-low heat. Set aside to cool slightly.
In a medium bowl, whisk together cornmeal, flour, baking powder, baking soda, pumpkin pie spice and salt. Set aside.
In a second medium bowl, whisk eggs until the yolks are incorporated into the whites. Add buttermilk, mashed sweet potato, maple syrup and melted butter; stir until smooth.
Add sweet potato mixture to cornmeal mixture. Fold ingredients together with a rubber spatula or wooden spoon until combined; don't over-mix.
Remove preheated skillet. Add oil to hot skillet and swirl to coat bottom and sides of pan. Pour batter into skillet, scraping down the bowl with a rubber spatula. Smooth top and place on the oven's center rack.
Bake for 25 minutes or until knife or toothpick comes out clean. Let cool 5 minutes or more before serving.