Preheat the oven to 400°F. Use 1 tablespoon of the butter to grease a rectangular or oval 2-quart casserole.
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 the 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 into 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.