Bring the milk to a simmer in a saucepan
In a bowl, add 1/3 cup of sugar to egg yolks, salt and corn starch, immediately beat the mixture vigorously with a whisk until smooth. If you let the sugar sit on the egg yolks without beating right away the egg yolks will crystallize.
Slowly pour the hot milk into the egg yolk mixture starting with about 2 tablespoons at a time, beating the yolks as you add the milk.
Pour the mixture back into the saucepan and cook over medium low heat, constantly stirring the mixture with a wooden spoon until the milk has become a thick custard.
Remove from heat but continue to stir for another 2 minutes.
Stir in butter and vanilla extract. Continue to stir with a wooden spoon until the butter is incorporated.
Pour the mixture into a Pyrex dish, cover with plastic wrap directly on the cream to keep a skin from forming.