You can substitute reconstituted orange juice for fresh; however, squeezing navel oranges only takes about 15 minutes and can make all the difference in this cake's fresh flavor.
Combine sugar and cornstarch in a 3-quart saucepan; gradually whisk in fresh orange juice. Whisk in lightly beaten eggs. Bring to a boil (5 to 6 minutes) over medium heat, whisking constantly.
Cook, whisking constantly, 1 to 2 minutes or until mixture reaches a pudding-like thickness. Remove from heat, and whisk in butter and grated orange rind. Cover, placing plastic wrap directly on curd, and chill 8 hours.