Mashed Potato Base: 6 large russet potatoes, peeled and quartered 3 tablespoons kosher salt 1/2 cup cold unsalted butter, cut in pieces salt to taste 1 pinch cayenne 3/4 cup milk 4 ounces Comte, Gruyere, or Cheddar cheese, shredded Au Gratin Potato Topping: 4 large russet potatoes, peeled and halved 2 teaspoons kosher salt freshly ground black pepper to taste 2 tablespoons melted butter 2 ounces C…