2 pounds waxy potatoes, such as Yukon Gold, peeled and quartered
1/2 cup olive oil
Freshly ground pepper
In a medium saucepan, cover the potatoes with cold water and bring to a boil over moderately high heat. Add 1 teaspoon of salt and boil until the potatoes are tender, about 20 minutes; drain. Return the potatoes to the pan and shake over high heat to dry them out. Mash the potatoes well. Add 1/4 cup of the olive oil and season with salt and pepper.
Preheat the oven to 400°. In a 9-inch heavy-bottomed ovenproof skillet, heat 2 tablespoons of the oil. Add the potatoes and smooth the top. Cook over high heat until a crust starts to form, about 3 minutes. Transfer the skillet to the oven and bake for about 20 minutes, or until the cake is browned and crusty on the bottom.
Set the skillet over high heat; shake the pan to loosen the cake. Invert the cake onto a cookie sheet. Heat the remaining 2 tablespoons oil in the skillet. Slide the cake into the skillet, crusty side up and cook over high heat for 2 minutes. Return the skillet to the oven and cook for about 10 minutes, or until the bottom is crusty. Quarter the cake and serve hot.