Whisk all the sauce ingredients together until smooth and creamy.
Heat a grill or skillet over medium high heat. Coat the chicken with the chili powder and a generous sprinkling of salt and pepper. When the grill is hot, add a little bit of butter if necessary to prevent sticking, and add the chicken. Cook for a few minutes on each side until the chicken is cooked through. I like to let it get it really golden brown on the outside. Set aside and let cool for at least five minutes. Cut or shred into small pieces.
In a mixing bowl, combine about half of the sauce with the chicken and the grapes. Add more sauce as needed to get everything evenly coated.
Stuff each butter lettuce leaf with the mixture. Add water or oil to the remaining sauce until it pours evenly off the end of a spoon – use it to drizzle over the top. Squeeze with lime juice when serving if you’re that kind of person.