Butter each slice of bread on one side, all the way to the edges then set aside.
Place one slice of prosciutto onto your work surface, then place one slice of buttered bread on top - butter-side down.
Top the bread with sliced mozzarella, then spread the basil pesto (2 tsp) roughly over the cheese.
Top with sliced tomato and season with salt and pepper, then top with an additional layer of sliced mozzarella if using.
Place second slice of buttered bread on top, butter-side up.
Place the second slice of prosciutto over the top of the sandwich, then fold the ends of prosciutto over each other to enclose.
Heat a panini press, then grill sandwich until cheese has melted and the prosciutto is crisp.