
Preheat the oven to 425 ̊. Mix the butter, olive oil, rosemary, salt and a few grinds of pepper in a large bowl.
Place a potato in the curved part of a wooden spoon. Make a series of deep cuts in the potato 1/8 inch apart, cutting almost all the way through. (The edges of the spoon will prevent you from cutting to the other side.) Repeat with the remaining potatoes.
Toss the potatoes with the butter mixture. Place cut-sides up on a rimmed baking sheet and bake until tender and crisp, 75 to 80 minutes. Gently shake the pan to coat the potatoes with the melted butter. Season with salt.