Prep the pan and oven: Heat oven to 400°F (200°C). Grease an 8-inch square pan or a 10-inch cast-iron skillet with butter.
Cook the sweet potato: Bake, steam, or microwave a peeled, cubed sweet potato until very tender.
Mash until smooth and measure out 1 cup. Let it cool slightly.
Mix dry ingredients: In a large bowl, whisk cornmeal, flour, sugar, baking powder, baking soda, salt, and cinnamon.
Mix wet ingredients: In another bowl, whisk buttermilk, eggs, melted butter, vanilla, and the mashed sweet potato until combined.
Combine: Pour wet into dry. Stir gently until just combined.
The batter should be thick but spreadable. Do not overmix.
Bake: Spread batter into the prepared pan. Bake 20–25 minutes, until the top is golden and a tester comes out clean with a few moist crumbs.
Make honey butter: While it bakes, beat softened butter with honey and a pinch of salt until fluffy and smooth.
Serve: Let the cornbread cool 10 minutes.
Slice warm and top with generous scoops of honey butter.