Prepare your mise en place.
Combine the milk, sugar, and pistachio paste in a medium heavy-bottomed saucepan over high heat. Bring to a boil, stirring frequently with a heat-proof spatula. Remove from the heat.
Combine the egg yolks and pastry cream powder in a small stainless-steel mixing bowl, whisking until completely smooth. Whisk in one-third of the hot milk mixture