Preheat Grill: Set the grill for medium heat; preheat for 5 minutes.
Prepare Spice Mixture: In a medium bowl, combine chili powder, paprika, salt, cumin, and garlic powder.
Prepare Foil Packets: Lay 2 16" X 12" pieces of heavy-duty foil on a flat surface. Spray foil with cooking spray.
Season Chicken: Toss chicken chunks in the remaining spice mixture until evenly coated.
Assemble Foil Packets: Divide seasoned chicken, peppers, onions and spice mixture among foil packets. Top each packet with 1 TBSP butter. Fold and seal edges of foil to create packets.
Grill Packets: Grill covered, rotating packets halfway through, until chicken is cooked through to an internal temperature of 160F / 71C and vegetables are tender, 18 - 22 minutes.
Prepare Margaritas:Mix the ingredients for the margarita until very well combined (tequila optional). Serve over ice if desired.
Serve: Carefully open foil packets and serve with your choice of accompaniments: (flour tortillas, sour cream, shredded cheddar, sliced avocado, lime wedges). Enjoy with margaritas.