Whisk the lemon juice,1/2 t. zest,sugar,cornstarch and cayenne in a bowl. Set aside.
Place salt and remaining zest in a food processor until finely ground. Whisk water and lemon salt in a large bowl until salt dissolves. Add chicken and refrigerate,covered for 30 mins or up to 1 hour.
Remove chicken from brine,pat dry. Heat oil in a large cast iron skillet over med-high heat until just smoking. Place chicken skin side down in skillet, cook until very brown and crisp,10-15 mins. Turn chicken over,reduce heat to medium and cook 5-10 mins. longer. Remove from pan and keep warm.
Pour off the fat,add orange juice mixture and bring to a boil over medium heat. PLace chicken skin side up ,simmer until the sauce is thick 2-3 mins. Turn chicken to coat with sauce. Serve hot.