Begin by patting the whole chicken dry with paper towels to remove excess moisture, which helps achieve crispy skin. Place the chicken breast-side down on a cutting board and use kitchen shears to remove the backbone, carefully cutting along each side. Flip the chicken over and press down firmly on the breastbone to flatten it; now it's ready for seasoning.

In a small bowl, combine brown sugar, paprika, garlic powder, black pepper, and kosher salt to create a seasoning mix. Rub this spice blend generously all over the chicken, ensuring that it gets into every nook and cranny. Let the seasoned chicken sit while you prepare the grill.

Heat your grill to medium-high, aiming for a temperature between 350°F and 400°F. Brush the grates with extra-virgin olive oil to prevent sticking. Once preheated, place the chicken skin-side down on the grill for about 5 minutes or until the skin is golden and crispy.

After achieving a beautiful sear, carefully flip the spatchcocked chicken over so the skin side is now facing up. Lower the grill temperature to low, adjusting it to between 250°F and 300°F. Close the lid and continue to cook the chicken until its internal temperature reaches 165°F, which should take around 45 minutes.

While the chicken is grilling, prepare the flavorful garlic herb sauce. In a blender, combine fresh basil, cilantro, green onions, garlic cloves, mayonnaise, fresh lemon juice, and any remaining salt. Blend until smooth and creamy, adjusting seasoning to taste.

Once the chicken reaches the desired temperature, remove it from the grill and let it rest for about 10 minutes. Carve the chicken into serving pieces and serve it alongside the vibrant garlic herb sauce.