Place potatoes, chiles, corn, onion, oil, chili powder, salt and pepper in large bowl; toss to coat. Arrange one-eighth of the vegetables on each piece of foil. Add 1 tablespoon stock and 1 teaspoon butter to each. Bring up 2 sides of foil so edges meet. Seal edges, making tight 1/2-inch fold; fold again, allowing space on sides for heat circulation and expansion.