Place 6 slices of bread on a large foil-lined baking sheet; top with 6 slices of the cheese. Divide sauerkraut mixture evenly atop cheese on bread slices. Top sauerkraut mixture with ham, if you like, and remaining 12 slices of cheese. Using a pastry brush, brush remaining 6 slices of bread with some of the butter. Place bread slices, buttered side up, atop cheese to make 6 sandwiches. Turn sandwiches; brush bread tops with remaining butter.