In medium saucepan, mix together lemon juice and cornstarch until smooth.
Add butter and sugar and bring mixture to boil over medium heat. Stir constantly for one minute. In small bowl use a whisk to beat egg yolks until smooth. Add in a small amount of the hot lemon mixture to the egg yolks. (this is tempering the eggs) Now pour the egg mixture back into the sauce pan, whisking rapidly until fully combined.
Reduce heat to low and stir constantly until reach desired thickness. Don't let it come to a boil again! (takes about five minutes)
Make sure its cool before adding to cake. Its a good idea to make this ahead of time so your cake assembly can be easier!