Heat griddle or 12-inch nonstick skillet over medium heat.
Spread butter on one side of each bread slice. Layer unbuttered side of 4 of the bread slices with 1/4 cup of the cheese, 2 slices of the prosciutto, 6 of the basil leaves and 2 of the tomato slices. Sprinkle with salt and pepper. Top each with 1/4 cup of the remaining cheese and remaining bread slice, buttered side up.
In hot skillet, heat sandwiches 3 to 5 minutes or until cheese is slightly melted and bread is browned. Turn sandwiches over; heat 3 to 5 minutes longer or until cheese is melted and sandwich is golden brown.