1. Using a sharp bread knife, slice the round ends off the boules, then cut each loaf into six equal slices for a total of 12 slices.
2. Heat an outdoor grill or grill pan to medium-hot. Liberally season the chicken with salt and pepper. Grill chicken breasts, turning once, until golden brown and no longer pink in the center, about 15 minutes.
3. Allow the chicken breasts time to cool, then slice in half horizontally to create six thinner pieces of chicken.
4. To assemble the sandwiches, spread each slice of bread with butter on one side.
5. On prep surface, place one slice of bread butter-side down and top with two slices of mozzarella, laid side by side, so the bread is entirely covered. Place two slices of tomato side by side atop the cheese, then top with a layer of arugula leaves. Finally, add one chicken-breast half, and place the second slice of bread on top, butter side up.
6. In a panini press or grill pan, cook the panini until the top is lightly browned and the cheese is melted. Slice in half and serve.

Makes 6 sandwiches.