Combine sugar, cornstarch, and salt in a medium saucepan; stir with a whisk. Stir in lemon juice and eggs. Bring to a boil over medium heat, stirring constantly with a whisk.

Reduce heat; simmer 1 minute or until thick, stirring constantly. Remove from heat. Add butter and lemon zest; stir until butter melts. Spoon mixture into a bowl; cool.

Cover and chill