Preheat oven to 375 degrees F. Place sweet potatoes in a 15x10x1-inch baking pan; prick each several times with a fork. Bake for 60 to 70 minutes or until very tender. Let cool. Cut each sweet potato in half lengthwise. Scoop out pulp, discarding skins. Transfer sweet potato pulp to a food processor. Cover and process until very smooth, scraping sides of bowl occasionally. Measure 3 1/2 cups sweet potato puree (save any remaining puree for another use).
In a 5-quart heavy pot combine sweet potato puree, brown sugar, honey, tangerine juice, the water, cinnamon, salt, vanilla bean paste, and nutmeg. Bring to boiling; reduce heat. Simmer, uncovered, about 15 minutes or until slightly thickened, stirring frequently (if mixture spatters, reduce heat further). Stir in tangerine peel.
Ladle sweet potato butter into half-pint canning jars, airtight storage containers, or freezer containers, leaving a 1/2-inch headspace. Cool for 30 minutes. Seal and label.
Store in the refrigerator for up to 1 week or freeze for up to 6 months.