Make sure oven rack is in the lower middle position and heat oven to 350F degrees. Coat 13×9 baking dish or dutch oven with cooking spray. Following package directions, cook entire box of pasta until just tender.
Mix cheeses and sour cream in a large bowl. Add the hot pasta and toss to coat thoroughly. Add salt and pepper to taste and transfer to casserole dish. Mix bread crumbs, parsley, and butter together in a small bowl, and sprinkle evenly over the shells and cheese (I forgot the parsley. I didn't miss it).
Bake 30-35 minutes until the casserole is bubbly and crumbs are golden brown. Don't share the leftovers with anyone.
Recipe by In Good Taste at https://ingoodtaste.kitchen/shells-and-cheese/