Pour buttermilk & salt in a large bowl, airtight container. Whisk or mix to combine. Add smashed garlic & spatchcocked chicken to buttermilk mixture, submerging the chicken as much as possible. Cover or seal, then transfer to the refrigerator to brine for up to 24 hours.
Add oil, lime zest & juice, honey, chili powder, paprika, garlic powder, dried oregano, & cayenne pepper (if using) to a small bowl or jar. Stir to combine & set aside, or store in an airtight container in the refrigerator for up to 5 days.
At least 60 minutes before grilling, remove chicken from the refrigerator (& the brine, if using). Use paper towel to pat chicken very dry. Place chicken breast-side up on a large plate or baking sheet. Season chicken liberally with S&P. Massage chili lime wet rub all over chicken, using your hands to work marinade between the skin & meat. Set aside to marinate.
Prepare your grill for indirect cooking, creating a zone of direct high heat & a second zone of indirect heat. If using a gas grill, begin preheating at least 15 minutes before grilling. If using charcoal, light the charcoal at least 30 minutes prior to grilling.
Place chicken on the indirect side of the grill, breast side up, as close to the fire as possible without being over the direct heat. Close grill lid. Cook chicken, stopping to rotate it 180 degrees every 15 minutes, until an instant-read thermometer inserted in the thickest part of the breast (without hitting the bone) registers a temperature of 160 degrees F. Depending on the size of your chicken & temperature of your grill, this could take as little as 25 minutes or up to 1 hour.
At some point while the chicken grills, place jalapeños in the direct heat zone of the grill, grilling 5-10 minutes until blistery & charred. Remove from the grill. Once cool enough to handle, peel the skin off the peppers & roughly chop. Place in a food processor with garlic, lime juice & kosher salt. With the food processor running, slowly stream in the grapeseed oil, until the sauce is thickened & creamy. Add the cilantro & pulse to combine. Set aside.
Move chicken over direct heat, carefully flipping it so its skin-side is down. Cook for 3-5 minutes, until the chicken skin is charred as desired.
Remove chicken from the grill & set aside to rest for 5-10 minutes. Carryover heat will cause the internal temperature to continue to rise as it rests. The chicken will be ready to serve once an instant-read thermometer inserted in the thickest part of the chicken (without hitting the bone) registers a temperature of 165 degrees F.