Wash potatoes well; rub with 2 tablespoons butter. Wrap each in heavy-duty aluminum foil, shiny side in. Seal tightly.
Knock gray ash off briquets or coals in grill. Lay potatoes on coals. Cook, turning frequently, for 50 to 60 minutes. Potatoes are done when they yield to soft pressure.
Open foil; slit tops and squeeze gently. Top with butter and salt and pepper to taste. Add sour cream, if desired.