oooh I just bought buttermilk yesterday to do the same thing with for Thanksgiving. Here's what I'm using mine for:
Smashed Crème Fraîche Sweet Potatoes
from Serious Eats
1 sweet potato, peeled and cubed into 1-inch pieces
1 baking potato, peeled and cubed into 1-inch pieces
1 tablespoon butter
2 tablespoons cream fraîche
1/4 cup (or more if necessary) half and half
Place the sweet potato and baking potato chunks into a pot of cold water. Bring to a boil, and cook until fork-tender, about 20 minutes from the boil.
Drain off the potatoes, and then return them to the same hot, but now dry, pot. Put the pot on a low heat for just about 1 minute, to allow any excess liquid to burn off. Add in the butter and crème fraîche and season with salt. Mash the potatoes with a potato masher. Then add the half and half until you have the consistency you desire. Mash a bit more, and you're done.
I usually double or triple this because it's soooo good and great as leftovers. I loved mashed 'taters but adding the sweet potatoes and the crème fraîche put them right over the top.
Editing to add the recipe I use for crème fraîche:
Combine 1 c whipping cream with 2 T buttermilk in a glass jar, cover and shake well. Let it stand at room temp for about 24 hours, until it's nice and thick. Stir it well and store it in the fridge. In theory it will last 10 days, but mine never does because we eat it too quickly.
