Preheat oven to 350°F (175°C) and grease a 9x9-inch (23x23 cm) baking dish. Set aside.
You will need to make mashed sweet potato by placing peeled and cubed sweet potatoes in a large pot of boiling water and cooking until they are fork-tender, about 10-15 minutes. (Alternatively, you can bake a whole sweet potato in the oven or slow cooker until fork tender, then remove the skin and mash.)
Drain the sweet potatoes and transfer them to a mixing bowl.
Mash the cooked sweet potatoes until they are smooth and free of lumps. You can use a potato masher or a hand mixer for this step.
Add 1/4 cup of unsalted butter to the mashed sweet potatoes and mix until the butter is completely melted and incorporated.
In a separate bowl, whisk together the sugar, eggs, vanilla extract, and milk until well combined.
Slowly pour the egg mixture into the sweet potato mixture, stirring continuously until everything is well incorporated. Set this sweet potato mixture aside.
In another bowl, combine the yellow cornmeal, baking powder, salt, ground cinnamon, ground nutmeg, and ground allspice.
Gradually add the buttermilk to the dry ingredients and stir until you have a smooth batter.
Now, fold the melted butter into the cornmeal batter, making sure it's well combined.
Gently fold the cornmeal batter into the sweet potato mixture until you have a uniform batter with no streaks of either mixture.
Pour the spoonbread batter into the greased baking dish, spreading it evenly.
Bake in the preheated oven for 45-55 minutes or until the spoonbread is set and the top is golden brown. You can test for doneness by inserting a toothpick into the center; it should come out clean when it's done.
Remove the spoonbread from the oven and let it cool for a few minutes before serving.
Serve your Sweet Potato Spoonbread warm as a side dish or a dessert. It's delicious on its own or topped with a dollop of whipped cream or a drizzle of maple syrup.