Toast 6 bread slices; spread dressing evenly over 1 side of each slice. Brush melted butter over 1 side of each slice of remaining untoasted bread. Place 6 bread slices, buttered sides down, on work surface; spread each with 1 teaspoon mustard. Divide pastrami evenly over mustard-coated bread slices; top each with 2 tablespoons sauerkraut, 1 cheese triangle and 1 toasted bread slice, dressing side up. Divide corned beef evenly over dressing-coated bread slices; top each with 2 tablespoons sauerkraut, 1 cheese triangle and remaining bread slices, buttered sides up. Secure sandwiches with toothpicks.