PREHEAT oven to 425°; SET oven rack at upper-middle position.
Using whole, unpeeled potatoes, PRICK skins with fork; LINE a baking pan with foil; PLACE potatoes on foil; BAKE until tender, about 45 to 60 minutes.
REMOVE from oven; allow to COOL; PEEL.
SLICE potatoes into chunks; PLACE in blender along with slat and pepper; PUREE, adding buttermilk, milk and butter gradually; PROCESS until silken.
Can make up to 2 days ahead: STORE in an airtight container; REHEAT; SERVE.
Per Serving (excluding unknown items): 220 Calories; 10g Fat (38.7% calories from fat); 3g Protein; 31g Carbohydrate; 4g Dietary Fiber; 26mg Cholesterol; 261mg Sodium. Exchanges: 2 Grain(Starch); 0 Non-Fat Milk; 2 Fat.