In a mixing bowl, whisk together the egg yolks and sugar until pale and slightly thickened.
In a medium saucepan, combine whole milk and the zest of the orange. Heat until just about to simmer, stirring occasionally.
Slowly whisk in flour or cornstarch into the egg yolk mixture to prevent lumps.
Gradually pour the hot milk into the egg mixture while whisking constantly to temper the eggs.
Return the mixture to the saucepan and cook over medium heat, stirring continuously until thickened (5-7 minutes).
Remove from heat and stir in fresh orange juice to taste.
Strain the pastry cream through a fine mesh strainer into a clean bowl and cover with plastic wrap to cool.
