I put this together on a whim, while cleaning out Thanksgiving leftovers from my refrigerator. I added the protein powder to sort of fortify the nutrition, but it's totally optional. I chose to use the eggnog for the liquid because it too was a leftover - but cream or evaporated milk will work just as well. The eggnog adds a unique flavor!
Preheat oven to 350°F. Place a large dish of hot water in the oven to preheat also. Be sure this dish will hold your pudding mold.
Spray a 1-quart deep-dish casserole or pudding mold with cooking oil.
Mash the cooked sweet potatoes well and place in a large mixing bowl. Beat in the other ingredients, about 50 strokes by hand or half a minute with an electric mixer, until all is well-blended.
Spoon pudding mixture into prepared mold, and place mold in the hot water bath. Bake for 75 - 90 minutes. The time will depend on how deep your pudding dish is - the shallower the dish, the less the cooking time. When done, it will be puffed on top and a knife inserted in the center will come out clean.
Remove from the hot water bath and cool pudding in the dish on a rack for 20-30 minutes. You may serve it after cooling as a side dish, or refrigerate 1-2 hours and serve cold as a dessert.