1. Combine first 3 ingredients; sprinkle evenly over both sides of chicken. Heat butter and oil in a large deep skillet over medium-high heat. Add chicken to pan; sauté 5 minutes on each side or until browned. Remove pan from heat. Pour gin into one side of pan; return pan to heat. Ignite gin with a long match; let flames die down. Remove chicken from pan; keep warm.
2. Add celery, carrot, shallots, and juniper berries to pan; sauté 5 minutes or until vegetables are tender, stirring occasionally. Add mushrooms. Place thyme, parsley, and bay leaf on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely. Add cheesecloth bag to pan. Return chicken to pan, nestling into vegetable mixture. Stir in beer; bring to a simmer. Cover, reduce heat, and simmer 45 minutes or until a thermometer inserted in the meaty parts of chicken registers 160°. (Breasts may cook more quickly. Check them after 35 minutes, and remove them when they're done; keep warm.)
3. Discard cheesecloth bag. Remove chicken from pan; keep warm. Place pan over medium heat; stir in yogurt. Cook 1 minute or until thoroughly heated (do not boil, as the yogurt may curdle). Remove from heat; stir in vinegar. Taste and adjust seasoning, if desired. Place 1 chicken breast half or 1 drumstick and 1 thigh on each of 4 plates; top each serving with about 3/4 cup sauce and vegetable mixture. Sprinkle with chopped parsley.