Using poultry shears or sharp knife, split the chicken along the backbone, leaving the breastbone intact, a preparation technique known as spatchcocking. Flatten the chicken by pressing down on it firmly. In a shallow, glass baking dish, mix together all ingredients for the marinade. Add chicken, rubbing marinade over entire surface. Cover the dish with plastic wrap and refrigerate for 20 minutes or up to 1 hour.

While chicken is marinating, prepare coals for the grill. Grill chicken over medium-hot coals for 30 to 35 minutes, until a thermometer inserted in the thickest part of the thigh registers 170 degrees F. Discard remaining marinade.

While chicken is grilling, whisk together all ingredients for vinaigrette in a small, glass or nonstick saucepan.

Transfer the grilled chicken to a cutting board and let cool for 5 minutes. Over low heat, warm vinaigrette, whisking well. Carve the chicken and pour vinaigrette into a gravy pitcher. Serve chicken warm and pass vinaigrette separately.