Step 1

Preheat oven to 375°F. Place sweet potatoes in a 15x10-inch baking pan; prick each several times with a fork. Bake 60 to 70 minutes or until very tender. Add Thai peppers to the baking pan the last 6 minutes of roasting time; roast until tender and lightly browned in spots. Let sweet potatoes cool. Cut each sweet potato in half lengthwise. Scoop out pulp, discarding skins. Remove stems from peppers. Place sweet potato pulp and peppers in a food processor. Cover and process until very smooth. Measure 1 3/4 cups puree (save any remaining for another use.)

Step 2

In a 5-quart heavy pot combine sweet potato puree and the remaining ingredients. Bring to boiling; reduce heat. Simmer, uncovered, about 20 minutes or until thickened and mixture mounds on a spoon, stirring frequently. (If mixture spatters, reduce heat.)

Step 3

Place pot of sweet potato-pumpkin butter in a sink filled with ice water; stir to cool. Ladle into clean, wide-top half-pint freezer containers, leaving a 1/2-inch headspace. Seal and label. Store in the refrigerator up to 2 weeks or freeze up to 6 months.