Caprese Panini starts with buttered sourdough bread which is grilled to perfection. It surrounds mozzarella cheese, tomatoes, basil, and a tangy balsamic glaze.
Spread the inside of 4 slices of bread with balsamic glaze.
Build sandwiches by adding mozzarella, then tomatoes, then basil leaves to two slices of bread (glazed sides facing in).
Top the sandwiches with the other two slices of bread (plain sides facing out).
Brush tops and bottoms of bread slices with melted butter.
Grill sandwiches in a cast iron skillet over medium heat until crispy and golden brown, turning once during grilling process.
If you wish, you can press the sandwiches with a panini press or heavy pan while cooking. This gives the sandwich a crispier crust.
Notes
If you can't find balsamic glaze in your store, make it by boiling 8 oz. balsamic vinegar and ¼ cup brown sugar together for 20 minutes over medium heat. Let it cool about 10 minutes before using, then store in the refrigerator.