The pastry cream can be made up to 3 days ahead of time and stored in an airtight container in the refrigerator.
In a small saucepan, heat the skim milk and 1 teaspoon of the orange zest over medium-high heat until the milk is scalded (remove the milk from the heat just before it begins to boil. Don't let the milk boil.)
While waiting for the milk to heat up, whisk the egg and egg yolk together (save the spare egg white for the meringue)
Mix 2 tablespoons of cornstarch with 3 tablespoons of sugar. Whisk the cornstarch and sugar mixture into the eggs.
Strain the orange zest out of the scalded milk. Pour the scalded milk through a strainer and into a large measuring cup.
Gradually pour about half the hot milk into the egg yolk mixture, whisking constantly. (You don't want to end up with scrambled eggs! Pour slowly, and whisk constantly.) We are "tempering" the eggs to avoid scrambling them.
Transfer the egg yolk mixture to the saucepan with the rest of the scalded milk. Cook over medium heat, whisking constantly, until the mixture begins to thicken to a pudding-like consistency. This should only take a minute or two. Don't allow the mixture to boil longer than a few seconds, or it will turn grainy.
Transfer to a bowl or airtight container. Prevent the pastry cream from developing a skin by laying plastic wrap directly on top of the pastry cream. Put it in the refrigerator to cool.