Heat the butter in a large sauté pan, over medium heat. Once the butter has melted, add the heavy cream and cook for about a minute, or until it starts to simmer. Stir in the parmesan cheese and cook until dissolved. Then, add the pesto sauce and stir until combined. Taste for seasoning and fix with salt and pepper.

Remove the pan from the heat and add tomatoes, mozzarella balls and the spaghetti. Toss everything together, adding a little reserved pasta water if necessary.

Transfer to a serving dish (or individual bowls), top with fresh basil leaves and serve immediately, with some freshly grated Parmigiano Reggiano on the side.