1. Combine sugar and cornstarch in a 3-qt. saucepan; gradually whisk in orange juice. Whisk in egg. Bring to a boil; boil, whisking constantly, 3 to 4 minutes.
2. Remove from heat; whisk in butter, zest, and salt. Place heavy-duty plastic wrap directly on curd (to prevent a film from forming), and chill 8 hours. Store leftovers in refrigerator up to 3 days.
Note: We tested with Simply Orange 100% Pure Squeezed Orange Juice.