Preheat oven to 450 degrees. Line a baking sheet with parchment paper and set aside.
Prick potatoes all over using the tines of a fork. Place potatoes on prepared baking sheet and roast until very soft, 45 to 60 minutes. Remove from oven and let cool slightly.
When potatoes are cool enough to handle, scoop out flesh and transfer to the bowl of a food processor; you should have about 1 1/2 cups flesh. Discard skins. Process sweet potato until smooth.
In a small bowl, whisk together miso and stock. Add to bowl of food processor with sweet potatoes; process until well combined. Transfer to a medium saucepan and place over medium heat; cook, stirring, until potatoes are heated through. Season with salt and pepper and serve immediately.