Heat oven to 400. Spray a lg baking dish w/ cooking spray. In shallow bowl whisk together, sour cream, mustard, garlic and pepper. Add chicken to bowl and turn to coat evenly.
Refrigerate chicken for 30 mins. In another shallow bowl, mix crushed cornflakes and onion soup mix. Gently press chicken into cornflake mix and place chicken in baking dish. Drizzle chicken w/ butter.