Peel potatoes and with a melon baller or French ball cutter, scoop out walnut-sized potato balls; drop into a bowl of cold water; let sit for 15 minutes.
Drain, place in saucepan with 1 tsp salt and water to cover; boil briskly for 5 minutes; drain.
Place potatoes, butter and olive oil in shallow baking pan and stir to coat potatoes; bake in preheated 350° oven until golden, turning to color evenly, about 20 minutes.