Preheat oven to 375° Fahrenheit. Line 8x8 baking dish with parchment paper, then lightly butter inner sides of dish not covered by parchment paper.
Add 1 cup cooked, mashed sweet potato, ½ cup plain yogurt, ¼ cup unsalted butter, ⅓ cup packed brown sugar, 2 large eggs, and 1 cup buttermilk to medium mixing bowl. Whisk ingredients together until fully incorporated. Set bowl aside.
Add 1 cup all-purpose flour, 1 cup yellow cornmeal, 1 tablespoon fresh baking powder, 1 teaspoon fresh baking soda, ½ teaspoon ground cinnamon, and ½ teaspoon salt to large mixing bowl. Stir ingredients together until well blended.
When dry ingredients are fully incorporated, transfer wet ingredients into bowl of dry ingredients. Use silicone spatula to stir ingredients together until just combined, making sure to break up any clumps of dry ingredients.
Transfer cornbread batter to prepared baking dish. Spread batter out evenly across baking dish, smoothing top of batter with spatula.
Place baking dish in preheated oven. Bake cornbread 20 minutes.
After 20 minutes, begin checking doneness of cornbread by inserting clean toothpick (or butter knife) directly into center of cornbread then immediately removing toothpick (or butter knife). Continue baking cornbread 5 to 10 minutes more, as needed, until toothpick (or butter knife) comes out completely clean, with no batter or crumbs.
When cornbread is baked through completely, carefully remove baking dish from oven. Set dish aside at let cornbread rest 10 minutes.