Step 1Heat the oven to 375 degrees. Put the sweet potato on a cookie sheet and bake until a knife easily pierces the meat to the center, about 1 hour. Meanwhile, generously butter 6 (one-half cup) straight-sided ramekins or individual souffle molds. Remove the potato from the oven and set aside until cool enough to peel.
Step 2Peel the potato and puree it in a food processor. Reserve two-thirds cup puree and save the rest for another use (1 large sweet potato weighs about 1 1/4 pounds and will yield about 1 1/2 cups of puree). Return the reserved two-thirds cup sweet potato puree to the food processor and add the ricotta, egg yolks, salt, nutmeg and pepper. Process to a smooth puree.
Step 3Roll the prosciutto slices into a tight bundle and slice into very thin strips. Scatter them with your fingertips to keep them from sticking together.
Step 4Beat the egg whites until frothy. Add the cream of tartar and continue beating until stiff peaks form. The egg whites will be ready when they form well-defined pointed peaks that stand straight up and retain their shape. 6. Add about one-half cup of the sweet potato puree to the egg whites and gently fold it in. The best way to do this is with a hand-held balloon whisk or with a spatula. Cut straight down through the puree and the egg whites, scrape the bottom and lift the egg whites over the puree. Turn the bowl a quarter turn and repeat. Keep doing this until the puree is fully incorporated, probably 3 or 4 times.
Step 5Add the remaining sweet potato puree and fold it in the same way. When you're done, the potatoes should be evenly distributed, but there may be small patches of egg white remaining. Do not over-mix or the egg whites will lose too much volume.
Step 6Scatter the prosciutto strips over the top and very gently fold them into the souffle mixture. Do not worry too much about them being distributed evenly.
Step 7Using a large soup spoon, evenly divide the souffle mixture among the buttered ramekins. Depending on how well the egg whites were beaten, you may have some excess batter, which can either be baked in another ramekin or discarded. (The recipe can be made to this point, covered tightly with plastic wrap and refrigerated for up to 2 hours or frozen for up to a month. Souffles can be baked straight from the refrigerator. If frozen, remove the plastic wrap immediately and warm at room temperature for 30 minutes before baking.)
Step 8Put a jellyroll pan in the heated oven and arrange the souffle ramekins on it. Bake until the tops are puffed and golden brown in spots, 18 to 23 minutes. A sure sign of doneness is when the souffle becomes extremely fragrant. Do not over-bake, or the center will be dry.
Step 9Remove and serve immediately.