1) In a medium saucepan, add in sugar and cornstarch. Slowly add in the milk.
2) Stir continously till the mixture smoothens.
3) Add in the lemon peel.
4) Let the mixture come to a boil, while continously stirring.
5) Boil the mixture for 1 minute.
6) Add in a little of the hot mixture into the egg yolks. Transfer the mixture back into the saucepan while stirring continously.
7) Let the mixture boil before taking the pan off the flame.
8) Add in the vanilla.
9) Cover the sauce and chill for a minimum of 3 hours.
10) Lastly, add in the rum. Stir well.
11) Serve the Custard Sauce garnished with chopped fruits or candied fruits, if desired.