Preheat oven to 350˚F.
Poke the potatoes on all sides with a fork. Arrange on a parchment- or foil-lined baking sheet. Bake in upper third of oven for 45 minutes to 1 hour, until very tender.
Place butter in a small bowl, and add the miso and scallions. Stir together until combined.
Slice the sweet potatoes down the middle and top with a spoonful of miso-scallion butter. Serve with additional miso-scallion butter on the side.