Spread butter on 1 side of each bread slice. Arrange 3 slices bread, buttered side down, on work surface, and place 2 slices cheese and 1 teaspoon of the basil leaves on each bread slice. Top with remaining 3 bread slices, buttered sides up. In 10-inch nonstick skillet, cook 3 sandwiches at a time over medium heat, turning once, until golden brown and cheese just melts. Cut each sandwich into 6 squares. Place 2 sandwich squares onto each skewer.