For the herb butter, in a small bowl combine butter, jalapeño, sage, oregano, garlic, and salt and pepper to taste and mix well. Cover and set aside.
To butterfly the chicken, place chicken, breast side down, on a cutting board. With kitchen shears, cut along either side of the backbone and remove; discard backbone. Pry chicken open and place bone side down. Flatten the chicken by pressing hard down the middle.
Coat grill rack with nonstick cooking spray. Preheat grill to medium-high indirect heat.
Gently loosen and lift the skin from the chicken breast. Spread half of the herb butter under the skin, being careful not to tear the skin.
Place chicken, skin side down, on grill rack over direct heat. Grill, covered, 10 minutes. Carefully turn chicken; grill, covered, 5 minutes more.
Move chicken to indirect heat, leaving skin side up. Baste with remaining herb butter. Grill, covered, 15 to 25 minutes or until an instant-read thermometer inserted into the thigh registers 165°F.
Transfer chicken to a serving platter. Let stand 10 minutes before carving.
