Place Buffalo Mozzarella Ball (1) in a single layer between several paper towels. Place a cutting board on top of the covered mozzarella and add 2 pounds of weight for 15 minutes, unopened soup cans work well. Doing this will remove the excess moisture and will prevent your Panini from becoming soggy while in the press.
Preheat the panini press to medium-high heat.
To make the pesto. Add to the bowl of a food processor, the Fresh Basil Leaf (2 cup), Garlic (2 clove), Pine Nuts (1/4 cup), and Parmesan Cheese (1/2 cup). Pulse while slowly adding Olive Oil (1/2 cup) until smooth and desired consistency. Add the Kosher Salt (1/2 teaspoon) and Ground Black Pepper (1/8 teaspoon) and pulse a few more times to incorporate.
Using a sharp bread knife, slice the Focaccia Bread (1) down the middle vertically and then horizontally, creating 4 servings. Slice each section in half to create a top and bottom, and lightly brush the outside with olive oil.
Oiled side down, liberally spread the pesto on the inside of each half. On the bottom, add the sliced mozzarella, Plum Tomato (4), and Arugula (1 bunch). Season with salt and pepper. Place the tops of the bread on each sandwich and place in the Panini Press.
Cook 3 to 5 minutes, or until the cheese has melted and the bread is golden brown. Transfer the Panini to a cutting board and cut in half. Serve immediately.