Crack eggs in a small bowl and whisk together. Set aside.
In a saucepan, combine sugar, cornstarch, and salt. Whisk in 2 cups of milk. Turn on heat to medium. Cook, whisking frequently. Once mixture has started to boil make sure heat is on low and whisk continuously until it has boiled 2 minutes.
Temper eggs by pouring a little bit of the boiled milk mixture from the saucepan into the egg bowl, whisking together with the eggs (this prevents your eggs from scrambling). Continue adding small amounts at least 4 - 5 times before pouring the tempered egg mixture from the bowl into the saucepan. Cook mixture 2 more minutes whisking frequently.
Take saucepan off heat. Add butter and vanilla. Whisk to combine. Let cool slightly.