First, bring a large pot of salted water to a boil and par boil the potatoes till just tender, about 10 minutes. Drain the potatoes well and place them a large mixing bowl. Add the oil, butter, garlic, rosemary and salt and pepper and toss to combine. Place the mixture in a shallow roasting pan. Bake at 350 for 30-40 minutes, tossing often, until golden.