Prick each potato twice with a fork and bake in a foil-lined shallow baking pan in lower third of oven until very tender, about 1 hour. Remove and cool slightly.
Halve potatoes lengthwise and scoop out warm flesh into a large bowl. Mash potatoes with a potato masher or, for a smoother purée, force through a potato ricer. Stir in butter, cream, syrup, salt, and pepper.
Cooks' note: Potatoes can be made 2 days ahead and chilled, covered. Reheat in a 350F oven or a microwave.