Place sweet potatoes on a rimmed baking sheet and roast for 45 minutes (or longer if your spuds are particularly large. Cook the sweet potatoes until they're soft inside and the skins are puffed away from the flesh. (You should be able to press the skin with your finger and the flesh beneath will feel soft.)
Set the roasted sweet potatoes aside to cool.
When the potatoes are cool enough to handle, cut a slit in the top of each and peel away the skin (it should come right off).
Transfer the flesh of the sweet potato to the food processor and add the butter.
Pulse several times, scraping down the sides if necessary to puree the flesh.
Add the buttermilk and kosher salt and pulse to combine. Taste for seasoning and adjust as necessary.