Add sugar and cornstarch to a medium sized pot and whisk to combine
Add juice, and zest and heat over medium high heat until soft boil
Stir well, reduce heat to medium low, and keep heating until thickened. Note that this can burn quickly around edge so watch carefully and stir continuously when thickening.
Remove from heat and stir in liquor after it has cooled.
Store in glass jar in the refrigerator.