Over MEDIUM heat, stir the milk, half and half and sugar until sugar is dissolved.
Separate the 4 large eggs. Whisk egg yolks and reserve egg whites for other uses.
Whisk a couple of tablespoons of hot milk into the beaten egg yolks. Repeat a couple of more times.
Add yolk mixture back into the saucepan with the rest of the milk over MEDIUM heat. Stir constantly until it begins to thinly coat the back of the spoon. DO NOT BOIL!
Strain, cool and whisk in vanilla.