In a food procesor pulse cornflakes until ground coarse, and spread in a shallow baking pan. In a large bowl whisk together mayonnaise, mustard, Old Bay seasoning, cayenne and yolks, and then add crabmeat, salt and pepper to taste, tossing mixture gently but thoroughly. With a 1/4-cup measure form a crab mixture into slightly flattened rounds about 2 1/2 inches wide and 3/4-inch thick and gently coat crab cakes with cornflakes, transferring as coated to a wax-paper-lined baking sheet. Chill crab cakes, covered with plastic wrap, at least 2 hours and up to 4 hours. Preheat oven to 400F. Transfer crab cakes to a large baking sheet. Put about 1/2 teaspoon butter on each crab cake and bake in middle of oven until crisp and cooked through, about 15 minutes. Serve crab cakes with tartar sauce. Makes about