Method:

Sauce

1. Combine jalapeños, cilantro, garlic, mayonnaise, sour cream, lime juice, and vinegar in the jar of a blender. Blend on high speed, scraping down as necessary, until smooth. With the blender running, slowly drizzle in olive oil. Season to taste with salt and pepper. Sauce will be quite loose at this point, but will thicken as it sits. Transfer to a sealed container and refrigerate until ready to use.

Chicken

1. Pat the chicken dry with paper towels and place on a large cutting board, breast side down. Using sharp kitchen shears, remove backbone by cutting along either side of it. Turn chicken over and lay out flat. Press firmly on breast to flatten chicken. For added stability, run a metal or wooden skewer horizontally through chicken, entering through one thigh, going through both breast halves, and exiting through the other thigh. Tuck wing tips behind back.
2. Combine salt, cumin, paprika, pepper, garlic, vinegar, and oil in a small bowl and massage with fingertips until homogeneous. Spread mixture evenly over all surfaces of chicken.
3. Light 1 chimney full of charcoal. When all charcoal is lit and covered with gray ash, pour out and spread coals evenly over half of the coal grate. Alternatively, set half the burners of a gas grill to high heat. Set the cooking grate in place, cover the grill, and allow to preheat for 5 minutes. Clean and oil grilling grate.
4. Place chicken, skin side up, on the cooler side of the grill, with legs facing toward the hotter side. Cover grill, with vents on lid open and aligned over chicken. Open bottom vents of grill. Cook until an instant-read thermometer inserted into the thickest part of breast registers 43°C. Carefully flip chicken and place, skin side down, on the hotter side of the grill, with breasts pointed toward the cooler side. Press down firmly with a wide, stiff spatula to ensure good contact between bird and grill grates. Cover and cook until skin is crisp and an instant-read thermometer inserted into the thickest part of breast registers 63 to 66°C, about 10 minutes longer. If chicken threatens to burn before temperature is achieved, carefully slide to the cooler side of the grill, cover, and continue to cook until done. Do not leave the lid off for longer than it takes to check temperature, or chicken will burn.
5. Transfer chicken to a cutting board and allow to rest for 5 to 10 minutes. Carve and serve with sauce.