In a saucepan over medium heat, mix 1 1/2 cups milk, 1/2 cup sugar, and vanilla. Bring to a boil.

While the mixture is heating, in a bowl combine the remaining milk and sugar and cornstarch. Stir well and whisk in the egg yolks.

Temper the egg yolks be adding about 1/2 cup of the hot milk, whisking constantly so it does not become scrambled eggs. Add the remaining milk and whisk. Return all of the mixture to saucepan and heat over medium heat, whisking continuously until the pasty cream comes to a boil and thickens in 5-7 minutes.

Remove from heat and add the butter, stirring until melted. Transfer the mixture to an ice bath and stir occasionally. It will be about 30 minutes in the bath.

Transfer to a container and place parchment paper directly on the pastry cream to keep a skin from forming and refrigerate. Will be good for 3 days.

Variations:
chocolate: add 1 1/2 cups chocolate chips into the hot mixture.
banana: mix in 2 chopped bananas after cooled in the ice bath.