Mix the Fajita seasoning in a small bowl and set aside.
Place the chicken breast in a plastic bag and beat with a rolling pin to flatten the breasts slightly. Remove the chicken from the bag and generously season both sides of the chicken. Generously season the chicken thighs. Reserve about 2 tablespoons of the seasoning.
Place the chicken breasts on a sheet of tin foil and place 2 tablespoons of butter on top of each breast and wrap up tightly. Place the thighs on a sheet of tinfoil and place 1 tablespoon of butter on top of each thigh, and wrap up tightly.
Place the chicken, butter side up, on a grill/smoker preheated to 400 degrees F. Shut the lid and cook for about 8 to 10 minutes, flip and cook for an additional 5 minutes.
Meanwhile, place a large cast iron skillet or pan on the grill with the chicken. Melt the remaining ½ stick of butter. Stir in the onion, bell peppers, jalapenos and reserved seasoning. Continue cooking, until the vegetables become tender, stirring occasionally.
Remove the chicken from the tin foil and place on the indirect side of the grill or smoker. I like to run the grill/smoker temperature around 350 degrees F. Cook the thighs for about 3 minutes per side, and the breasts about 8 minutes per side, or until the internal temperature is165 degrees F.
Remove the chicken from the grill/smoker and let cool slightly. Slice and stir into the pepper mixture. Simmer for about 5 minutes, stirring occasionally.
Spoon onto tortillas and top with your favorite toppings.