Whisk orange juice, 1/2 teaspoon orange zest, sugar, cornstarch and cayenne in medium bowl until no lumps remain; set aside.
Process salt and remaining zest in food processor, or grinder until finely ground. Whisk water and orange salt in large bowl until salt dissolves. Add chicken and refrigerate, covered, for 30 minutes or up to 1 hour.
Cover bottom of Dutch oven with aluminum foil. Remove chicken from brine and discard leftover brine. Pat chicken dry with paper towels. Heat oil in large non-stick skillet over medium-high heat until just smoking. Place chicken skin side down and weight with prepared Dutch oven. Cook until very well browned and crisp, 10 to 15 minutes.
Remove pot and flip chicken skin-side up. Reduce heat to medium and cook until chicken registers 160 degrees, 5 to 10 minutes. Transfer chicken to plate and let rest 5 minutes.
Pour off fat from pan and discard. Add orange juice mixture to now empty skillet and bring to a boil over medium heat. Return chicken, skin side up, and any accumulated juices to skillet and simmer until sauce is thick and glossy, 2 to 3 minutes. Turn chicken to coat. Serve