Pour butter into a 9x13-inch baking dish. Add chicken. Top with the peas and carrots, and the corn. Sprinkle with the pepper.
Stir together the biscuit mix, seasoning packet from the mix, ½ cup cheddar, and milk. Spread over the veggies (do not stir).
Whisk together the cream of chicken soup, chicken stock, and sour cream. Pour over the biscuit layer (do not stir). Sprinkle with remaining cheese. Bake uncovered for 45-55 minutes or until only slightly jiggly in the center.
Remove from oven and let stand for 10 minutes before serving.