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1
Run potatoes through the large shredding disk of a food processor. (If using a box grater, shred potatoes lengthwise so you are left with long shreds.).
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2
Place potatoes in a large bowl and fill with cold water. Using hands, swirl to remove excess potato starch, then drain in strainer.
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3
Transfer half of potatoes from strainer to the center of a clean kitchen towl. Gather ends together and twist as tightly as possible to expel maximum moisture.
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4
Wipe bowl dry. Transfer potatoes to bowl and repeat process with remaining potatoes.
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5
Sprinkle salt, cornstarch, and pepper to taste over potatoes. Using hands or fork, toss ingredients together until well blended.
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6
Melt half of the butter (2 tablespoons) in a 10-inch nonstick skillet over medium heat. When foaming subsides, add potato mixture and spread into even layer. Cover and cook 6 minutes. Remove cover and, using spatula, gently press potatoes down to form round cake. Cook, occasionally pressing on potatoes to shape into uniform round cake, until bottom is deep golden brown, 4 to 6 minutes longer.
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7
Shake skillet to loosen roesti and slide onto large plate. Add remaining 2 tablespoons butter to skillet and swirl to coat pan. Invert roesti onto second plage and slide it, browned side up, back into skillet. Cook, occasionally pressing down on cake, until bottom is well browned, 7-9 minutes. Remove pan from heat and allow cake to cool in pan for 5 minutes. Transfer roesti to cutting board, cut into 4 pieces, and serve immediately.
