Spread butter on 1 side of each bread slice. Arrange 6 slices bread, buttered side down, on work surface, and place 2 slices cheese and 2 basil leaves on each bread slice. Top with remaining 6 bread slices, buttered sides up. In 10-inch nonstick skillet, cook 2 sandwiches at a time over medium heat until golden brown and cheese just melts; repeat for remaining sandwiches. Serve with soup.