In a medium-large bowl cream together the sugar and butter until fluffy.
Add the eggs and mix well again. If you desire a thick curd, mix in the cornstarch as well but it is not required.
Pour in the vanilla if desired and the lemon juice, mix well.
Immediately move to a small saucepan and cook over medium-low heat (just below boiling while stirring) for 10-12 minutes stirring constantly. Make sure to scrape the bottom and sides while stirring.
Once removed from heat cool completely before transferring to a glass container. Can be stored in the fridge for 1-2 weeks.