In a saucepan over medium heat, combine the milk, sugar, cornstarch, and flour. Whisk until smooth.
Stir continuously until the mixture starts to thicken.
Reduce heat to low and stir in the egg yolk and vanilla extract.
Add the butter and stir until completely melted and the mixture is creamy.
Remove from heat and pour into serving glasses or bowls.
Let cool slightly, then refrigerate for at least 2 hours before serving.