Place shredded potatoes in large bowl; fill with cold water. Swirl to remove excess starch; drain in strainer.
Wipe bowl dry. Place one third of potatoes in a mound in center of clean kitchen towel. Gather towel, twist tightly and squeeze to expel maximum moisture. Transfer potatoes to bowl; repeat with remaining potatoes.
Sprinkle salt, cornstarch, and pepper to taste over potatoes. Toss to evenly combine.
In 12-inch non-stick skillet, melt 2 1/2 tablespoons butter over medium heat. Add potato mixture and form an even layer. Cover; cook 6 minutes. Uncover; using spatula, gently press potatoes down to form round cake. Continue cooking, occasionally pressing on potatoes, until bottom is deep golden brown, about 8 - 10 minutes.
Loosen roesti by shaking skillet; slide onto large plate. To empty skillet, add remaining 2 1/2 tablespoons butter, make sure pan is coated. Invert roesti onto second plate then slide it, browned side up, back into skillet. Cook, occasionally pressing down on cake, until bottom is well browned, 8 - 10 minutes. Remove pan from heat and allow cake to cool in pan for 5 minutes. Transfer roesti to cutting board, cut into 6 pieces, and serve immediately.