Place the egg yolks and sugar in a medium bowl and whisk until pale yellow. Place the milk in a heavy-bottom saucepan and bring just to a boil. Slowly whisk the hot milk into the egg/sugar mixture. Transfer the mixture back to the saucepan and stir over medium heat until the custard thickens, a few minutes. It should be thick enough to coat the back of a spoon. Stir in the vanilla. Transfer to bowl or serving pitcher.
Serve warm over apple cake.
Recipe by Flavor and Friends at http://flavorandfriends.com/irish-apple-cake-custard/