Preheat the oven to 400 F. Use 1 tablespoon of the butter to grease a rectangular or oval 2-quart casserole dish.
Prick the sweet potatoes with a fork and bake them for about 55 to 60 minutes, until they are very soft. Remove the potatoes from the oven, and let sit on a wire rack until cool enough to handle.
While the potatoes are cooling, combine the milk, maple syrup, salt, pepper, cloves, and cayenne in a medium-sized saucepan. Bring to a simmer over medium heat. Sprinkle in the cornmeal slowly, whisking all the while, until the cornmeal is all added. Continue to whisk over the heat until the mixture has thickened, about 4 minutes. Remove from the heat. Cool for about 10 minutes, until almost room temperature.
Peel off the skins and place the potatoes in a bowl. Mash them with a potato masher until fairly smooth (or use a ricer or a fork if that’s what you have). Stir the remaining 3 tablespoons of butter and the mashed sweet potatoes into the cornmeal mixture. Stir in the baking powder, and then stir in the egg yolks. The batter will be quite thick.
In a bowl, using a whisk or an electric mixer, beat the egg whites until they just form stiff peaks. Gently fold the egg whites into the sweet-potato cornmeal mixture, just until barely incorporated (you should still see a steak or two of egg white).
Gently transfer the batter to the prepared dish and bake for 25 to 35 minutes (depending on how deep the dish is) until puffed and golden brown. The middle will still have the slightest jiggle when you wiggle the pan. Serve hot.
Notes
When you whip the egg whites, don't beat them too much which will allow the whites to start to break down.
Take your time folding the beaten egg whites into the potato mixture. Stir in about ¼ of the mixture into the potatoes first to lighten them up, then gently fold the rest into the sweet potatoes. This allows them to stay fluffy and not deflate once blended into the heavier sweet potato and cornmeal mixture.
Serve the spoonbread as soon as possible once you take it from the oven to preserve as much puffiness as possible (but know that it will fall fairly quickly; that's ok!).