The following text and recipe are from Epicurious online:
If you've never had pale-fleshed Japanese sweet potatoes before, you'll be surprised by their subtler, drier flesh, which tastes unmistakably of chestnut. A bit of miso mixed into the scallion butter stealthily rounds out the interplay of sweet and umami that will have you eating all the way through to the last flaky remnants of skin.
8 small slender Japanese sweet potatoes 1 1/2 sticks unsalted butter, well softened
1 1/2 Tbs. miso paste (preferably white)
3 Tbs. finely chopped scallion
Preheat oven to 450F with rack in upper third.
Prick potatoes all over with a fork and put on a foil lined large baking sheet. Bake until very soft when squeezed, 45 minutes to 1 hour.
While potatoes bake, stir together butter, miso, and scallion until combined.
Slit hot potatoes lengthwise and, using oven mitts, push in sides to puff up potato. Serve with some scallion butter in center of each and with additional scallion butter on the side.
Caged off of the Epicurious website