In a medium sauce pan, heat the milk on low heat and whisk in the corn starch and sugar.
Turn the heat to medium and stir every few minutes until the mixture begins to thicken some.
In a small bowl, whisk the egg yolks together.
Add a few ladles full of the hot milk mixture to the egg yolks to temper them.
Be sure to whisk while adding the milk so the eggs don't clump.
Slowly pour the egg mixture into the pan of milk while whisking.
Continue to cook and whisk over medium heat until thickened (the mixture should coat the back of a wooden spoon).
Pour the mixture into a bowl through a sieve (colander).
Add add the lemon juice, salt and butter.
Allow to cool to room temperature then refrigerate until ready to serve. (about 4 hours)