Heat the oven to 350 F.
Place the potatoes in a large pot, then add enough water to cover by 1 inch. Salt the water, then bring to a boil. Cook until tender when pierced with a fork, then drain and transfer to a sheet pan. Bake for 10 minutes.
Using a food mill or a ricer, process the potatoes into a large bowl. Stir in 3/4 cup of the reserved buttermilk, 2 tablespoons of the mustard butter and salt, to taste. Serve with additional mustard butter on the side.