Combine ¼ cup kosher salt and ¼ cup granulated sugar with 2 quarts of cold water to prepare the wet brine, stirring until fully dissolved. Puncture the chicken breasts all over using a fork, place them in a Ziplock bag, and gently pound them to a uniform thickness of ½ to ¾ inch with a rolling pin or tenderizer. Add the prepared brine to the bag, seal tightly, and allow the chicken to soak for 30 minutes at room temperature or refrigerate for up to 24 hours. In a separate bowl, mix together the salt, black pepper, cayenne, garlic powder, smoked paprika, onion powder, and brown sugar to form the seasoning blend. Once brined, remove the chicken breasts, rinse thoroughly, and pat dry using paper towels. Lightly drizzle with olive oil, then coat evenly with the spice rub on all sides. Preheat the grill to 475°F. For indoor grilling, it is recommended to place the chicken on the grill while it preheats to help the interior reach temperature without burning the exterior. If chilled, allow the chicken to sit at room temperature for at least 10 minutes prior to grilling. Grill the chicken for approximately 5–7 minutes per side, ensuring the internal temperature reaches 155–160°F before removing it from the heat. The chicken will finish cooking as it rests and must reach a final temperature of 165°F; return to the grill briefly if necessary.