1. Preheat oven to 375°.
2. Peel outer skin from garlic heads, and discard. Cut off top one-third of each garlic head. Separate garlic heads into 40 cloves. Reserve any remaining cloves for another use. Place 35 cloves in center of a piece of heavy-duty aluminum foil; coat garlic with cooking spray. Fold foil over garlic, sealing tightly. Set aside garlic packet and remaining 5 cloves.
3. Place chicken, breast side up, on a rack in a shallow roasting pan. Place 5 reserved garlic cloves in cavity. Place garlic packet on rack in roasting pan. Bake, uncovered, at 375° for 20 minutes. Pour wine over chicken, and bake 40 minutes, basting occasionally with pan juices.
4. Remove garlic from pan; let cool 10 minutes. Bake chicken 30 minutes or until done. Remove and discard papery skin from garlic. Squeeze pulp from garlic cloves into container of a mini food processor. Add 2 tablespoons broth; process until smooth, stopping once to scrape down sides.
5. Combine remaining 1/2 cup broth, evaporated milk, and cornstarch in a small saucepan, stirring until smooth. Cook over medium-high heat 2 to 3 minutes, stirring occasionally. Stir garlic mixture and pepper into broth mixture. Serve chicken with garlic sauce.