In a medium saucepan, bring the milk, 1/4 cup sugar, butter, salt and vanilla to a gentle boil. Remove form the heat.
In a medium size bowl, whisk the egg yolks, 1/4 sugar and cornstarch.
In small amounts, whisk a little of the hot milk into the egg bowl. This is to temper the eggs yolk. Once the bottom of the bowl is warm, slowly whisk in the remaining warm milk.
Pour the mixture back into the saucepan and whisk constantly over medium-low heat about 2-3 minutes until it thickens and becomes a pudding texture.
Transfer the pastry cream to a container. Press a piece of plastic wrap onto the surface of the pastry cream to prevent a skin from forming. Refrigerate.