Place the sweet potatoes in a large pot and cover with water. Bring to a boil.
Reduce heat and simmer until the sweet potatoes are fork-tender, about 15-20 minutes.
While the sweet potatoes are cooking, prepare the Miso Butter by mixing the butter and miso paste in a small bowl.
Drain the sweet potatoes and return them to the pot.
Mash the sweet potatoes and gradually add the warmed vegetable broth and remaining butter.
Stir in the black pepper and season with salt to taste.
Transfer to a serving bowl and top with Miso Butter, allowing it to melt into the potatoes.