Heat the oven to 400°F. Swirl the oil in a 10-inch cast-iron skillet and warm it in the oven.
Whisk cornmeal, flour, sugar, baking powder, baking soda, salt, and cinnamon in a large bowl.
Whisk buttermilk, mashed sweet potato, eggs, melted butter, honey, and vanilla in a second bowl until smooth.
Fold the wet mixture into the dry just until combined; do not overmix.
Carefully pour the batter into the hot, oiled skillet and smooth the top.
Bake 20–25 minutes, until golden and a toothpick in the center comes out clean.
Cool 10 minutes in the skillet. Slice warm and serve.