- In a saucepan over medium-high heat, combine the remaining milk with the butter and sugar and bring it to a boil.
- Whisk the reserved cornstarch-milk-egg mixture into the boiling milk. Continue whisking energetically until it returns to a boil and thickens. Remove from the heat, and keep whisking for another minute.
- It is very important to cool the mixture as rapidly as possible. The best method is to spread it out on a clean metal sheet pan, cover it with plastic wrap, and put it into the coldest part of the refrigerator. Keep refrigerated until ready to use, but don't freeze it.