Put milk, tapioca and sugar in 2 quart microwavable bowl.
Microwave on high for 12 minutes, stirring occassionally.
Meanwhile, put the egg yolk in a small bowl and stir until smooth. When the milk mixture is finished, slowly drizzle 1 cup or more of the hot milk into the yolk while beating the yolk. Then pour the egg mixture back into the milk bowl and stir till combined.
Microwave the milk and egg mixture on high for 2 minutes. Then stir in the vanilla and butter.