Murasaki sweet potatoes with miso scallion butter

3 lbs Murasaki sweet potatoes
about 8 small pieces
1 stick unsalted butter
2 tbsp white miso paste
2 tbsp scallion
finely chopped, about 1-2 stalks
Let's face it ... in this recipe, the murasaki sweet potato is simply a vessel for the miso butter! ;) This super simple recipe is a crowd-pleaser. We often whip it up when entertaining guests.
Soften the butter overnight or a few hours ahead of time.
Preheat oven to 450°F with rack in the upper third.
Wash the potatoes then prick them all over with a fork and transfer onto 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 well-combined.
For the Murasakis to be enjoyed immediately, slit them lengthwise and, using oven mitts or a clean dish towel, push in the sides to puff up and scrunch the potato. Serve with some scallion butter in center of each and with additional scallion butter on the side.
To prepare Murasakis for children or to freeze, cool the sweet potatoes until their skin shrinks away from the flesh. Peel then cut into cubes. In a large bowl, mix with the miso butter with a spatula. Serve or transfer into freezer containers.
source: adapted from Epicurious but originally from Gourmet November 2007
The miso-scallion butter can be made 4 days ahead and chilled, covered. Soften at room temperature and stir before using. The sweet potatoes can be roasted (but not cut) 4 hours ahead and kept at room temperature, covered with foil. Reheat potatoes on a baking sheet on middle rack of a 350°F oven until heated through, about 20 minutes.