In a large bowl, dissolve the kosher salt and brown sugar in warm water. Submerge the chicken breasts completely. Cover and refrigerate for at least 30 minutes, up to 2 hours. Do not exceed 2 hours or the chicken may become too salty.

While the chicken brines, make the glaze. In a small saucepan, combine the barbecue sauce, apple cider vinegar, smoked paprika, garlic powder, and cayenne (if using). Simmer over low heat for 5 minutes, stirring occasionally. Remove from heat and set aside.

Remove the chicken from the brine, rinse under cold water, and pat very dry with paper towels. Let the chicken rest on a plate at room temperature for 10 minutes. Drying is critical—moisture on the surface creates steam, not sear.

Preheat your grill for two-zone cooking: one side high heat (450-500°F), the other side medium heat (350-375°F). Clean and oil the grates thoroughly.

Place the chicken on the hot side of the grill, lid open. Sear for 3-4 minutes without moving, until deep grill marks form and the chicken releases easily. Flip and sear the other side for 3 minutes.

Move the chicken to the cooler side of the grill (medium heat). Close the lid. Cook for 6-8 minutes, flipping once halfway, until the internal temperature reaches 155°F in the thickest part of the breast.

Brush the glaze generously onto both sides of the chicken. Close the lid and cook for 1-2 minutes more, until the glaze is sticky and the internal temperature hits 160°F. Carryover cooking will bring it to 165°F as it rests.

Transfer the chicken to a clean plate. Tent loosely with foil and rest for 5 minutes. This step is non-negotiable—it locks in juices. Slice against the grain and serve with extra glaze on the side.