Put the potatoes in a medium saucepan, add 4 cups of water and 1 teaspoon salt and bring to a boil. Simmer, partially covered, until nearly tender. Drain and pat dry.

Heat a 10-inch nonstick skillet. Add the vegetable oil and 1 tablespoon of the butter. When the foam subsides, add the potatoes and cook, tossing, until lightly golden. Season with salt and pepper and roughly mash the potatoes with a spatula. Dot the remaining 1 tablespoon of butter over the potatoes, cover and cook until crusty on the bottom, about 3 minutes. Slide the potatoes onto a plate and invert them back into the pan. Cook until the underside is good and crusty. Slide the potatoes onto a platter and serve.