Mix sugar & cornstarch in medium saucepan, gradually stir in water
Cook over medium heat, stirring constantly till mixture comes to a full boil.
Boil & stir 1-2 minutes. Gradually stir at least half of hot mixture into egg yolks.
Blend it back into the pan. Boil & stir 2-3 minutes. ( if you don’t your filling
will taste eggy). Remove from heat add butter lemon rind and lemon juice, mix well. Cool before using as cake filling , if using as pie filling you can pour it into a baked pie crust immediately.