Begin by patting the spatchcocked chicken dry with paper towels to ensure a crispy skin. Place the chicken breast side down on a clean cutting board, then carefully remove the backbone using sharp kitchen shears. Flatten the chicken by pressing firmly down on the breastbone until you hear a gentle crack.

In a medium bowl, mix together brown sugar, paprika, garlic powder, freshly cracked black pepper, and kosher salt until well combined. Generously rub this spice mixture over the entire surface of the chicken.

Preheat your grill to medium-high heat, aiming for a temperature range of 350-400°F. Use a paper towel dipped in extra-virgin olive oil to coat the grates.

Carefully place the chicken skin-side down on the grill over direct heat. Grill for around 5 minutes, then flip the chicken and reduce the grill heat to low (250-300°F). Cook for an additional 45 minutes, monitoring the internal temperature until it reaches 165°F.

While the chicken is grilling, assemble the creamy herb sauce. In a blender, combine fresh basil leaves, packed cilantro leaves, chopped green onions, garlic cloves, mayonnaise, fresh lemon juice, and a pinch of salt. Blend until smooth.

Once the chicken has reached the desired temperature, remove it from the grill and let it rest for 10 minutes. Carve as desired and serve with the herb sauce.