Ingredients for 8 servings:
1. 8 small japanese sweet potatoes (4 to 5 pounds total) or 8 small garnet sweet potatoes (4 to 5 pounds total)
2. 3/4 cup unsalted butter, well softened
3. 1 1/2 tablespoons miso (preferably white)
4. 3 tablespoons finely chopped scallions

Directions:
1. Preheat oven to 450°F with rack in upper third.
2. Prick potatoes all over with a fork and put on a foil-lined large baking sheet.
3. Bake until very soft when squeezed, 45 minutes to 1 hour.
4. While potatoes bake, stir together butter, miso, and scallion until combined.
5. Slit hot potatoes lengthwise and, using oven mitts, push in sides to puff up potato.
6. Serve with some scallion butter in center of each and with additional scallion butter on the side.
7. Scallion butter can be made 4 days ahead and chilled, covered.
8. Bring to warm room temperature and stir before using.
9. Sweet potatoes can be roasted (but not cut) 4 hours ahead and kept at room temperature, covered with foil.
10. Reheat potatoes on a baking sheet on middle rack of a 350°F oven until heated through, about 20 minutes.