In a food processor, combine the basil leaves, pine nuts, Parmesan cheese, and minced garlic. Pulse until finely chopped. Slowly drizzle in the olive oil until the desired consistency is reached. Season with salt and pepper to taste. Set aside.

Preheat a panini press or grill pan over medium heat. Spread a generous amount of basil pesto on one side of each slice of bread.

On 4 slices of bread, layer the buffalo mozzarella slices evenly. Top with slices of tomato and a sprinkle of salt. Add baby arugula on top.

Drizzle balsamic vinegar or glaze (if using) over the arugula. Place the remaining slices of bread pesto side down on top of the sandwiches to close them.

Brush the outer sides of each sandwich with olive oil. Grill for 3-4 minutes or until the bread is toasted and cheese is melted. If using a grill pan, press down with a spatula.

Once golden brown, remove from heat and let cool for a minute. Cut each sandwich in half diagonally. Serve warm.