Place potatoes in a large stock pot. Fill the pot with cold water until the potatoes are fully submerged and the top layer is covered in at least 2" of water. Pour 2 tbsp of salt into the pot.
Place the pot on the stove and bring the water to a boil. Once boiling reduce the heat to a simmer and cover the pot
Allow the potatoes to simmer for at least 10 minutes - potatoes should be soft when pierced with a fork.
Remove the pot from the stove and drain the water. Immediately return the hot potatoes to the stockpot and cover the pot to retain the heat.
Place a stick of butter, half and half and sour cream in the warm pot. Cover the pot and wait a few minutes for the butter to melt
Using an electric hand mixer, mix the potatoes until almost fully whipped. Best if served immediately!
Notes
Salt the water well. Two tablespoons in the pot isn't a typo — it's the only chance the potatoes themselves get to take on seasoning. Taste and salt again at the end.Don't over-whip. Low to medium speed, 30 to 60 seconds, and stop the moment the lumps are gone. Working them longer releases more starch and turns them gluey. Never use a food processor or blender.Scaling: 5 lb serves 12 at about ½ lb per person. For 6, halve everything; for 24, double it and use two pots so the potatoes aren't crowded.Make ahead: whip them completely, transfer to your reheating dish, dot the top with butter, pour over a splash of half and half, cover tightly and refrigerate up to 3 days.Reheat without the oven: stovetop over low heat with a splash of warm half and half, stirring, about 15 to 20 minutes. Or slow cooker on LOW for 2 to 3 hours, then WARM until dinner.Keep warm: a buttered slow cooker on WARM holds them for about 4 hours. Stir every 45 minutes and loosen with warm half and half if they tighten.