1.Mix the cornstarch, flour and 2 tablespoons sugar in a small bowl. In a large bowl add egg yolks and stir well.
2.Add milk and 2 tablespoons sugar into a small pot. Heat on medium, stirring occasionally until sugar dissolves. Do not let milk foam, a few bubbles here and there are ok.
3.Whisk the small bowl of dry ingredients into egg yolks and mix into a paste. Add the warmed milk 1/4 cup at a time, whisking the whole time. Gradually adding the same amount of milk until it is all in.
4.Return the mixture back into the pot. Heat over medium low heat stirring constantly until thickened, about 5 minutes.
5.Turn off heat and add butter and vanilla then mix well. Push custard through a sieve into a container. Place plastic wrap on top of custard and chill in the fridge for at least 2 hours prior to using.