1. In a saucepan on medium heat, add and combine sugar, cornstarch and salt. Pour water and mix all the ingredients until thick.
2. Take 1 tablespoon of the above mixture and mix it with the beaten egg just until combined. Then pour it back into the saucepan and mix well. Stirring continuously cook this for 2 minutes over medium heat.
3. Once done, remove from heat and stir in lemon juice, lemon rind and butter. Stir well until the butter melts.
4. once done, transfer it into a bowl and cover the bowl with a plastic wrap making sure to touch the surface of the lemon filling. Let this cool completely before use.