1. Bring a large pot of salted water to a boil for pasta. Meanwhile,
heat grill to medium; lightly oil grates. Season chicken with salt and
pepper. Grill until cooked through, 2 to 3 minutes per side. Remove
from grill, and cut into thin strips, halving if long.
2. Cook pasta until al dente, according to package instructions.
Reserve 1 cup pasta water; drain pasta, and return to pot. Add chicken,
tomatoes, mozzarella, parsley, Parmesan, and butter. Toss to combine.
Add reserved pasta water a little at a time to create a sauce that
coats shells (you may not need all the water). Serve with more Parmesan.