Preheat the oven to 425 degrees Fahrenheit. Place 1 Tablespoon of butter in a 10-inch cast iron skillet. Place the skillet in the oven to melt the butter.
Meanwhile, combine cornmeal, all-purpose flour, baking powder, baking soda, ground cinnamon, and salt in a large bowl and stir to combine.
In another bowl, whisk buttermilk, brown sugar, eggs, and melted butter together. Add mashed sweet potatoes and stir until combined.
Add wet mixture to dry mixture and whisk until smooth.
Pour into the prepared baking dish. Bake in preheated oven on the middle rack for 20-25 minutes, until the cornbread is golden and a toothpick inserted comes out clean.