Whisk the cornstarch and 1/2 cup milk in a small bowl until smooth. Whisk the eggs, sugar and salt in a medium saucepan; whisk in the cornstarch mixture and the remaining 1 cup milk until smooth.
Cook the mixture over low heat, stirring, until thick enough to coat a spoon, 7 to 8 minutes (do not boil). Pour through a fine-mesh sieve into a bowl, then set the bowl in a larger bowl of ice water. Add the cream and stir until cool. Remove from the ice bath; stir in the vanilla, orange juice concentrate and orange zest. Cover and chill until cold.
Freeze the custard in an ice cream maker according to the manufacturer's directions. Scrape into a pastry bag fitted with a large star tip and pipe into cups.