Fill a small saucepan with 2 cups water and bring to a boil. Put the pistachios
in the water for 1 minute. Drain and rinse with cold water. Remove the skins
from the nuts.
Put the pistachios in a food processor with 1/2 cup of the sugar.
Process to a powder.
In a large bowl, whisk together the egg yolks and 1/4 cup of sugar.
Add the pistachio mixture and blend well.
Put the milk and lemon zest in a saucepan over medium heat and bring to a
boil. Add 1/2 cup of the hot milk to the bowl with the pistachio mixture. Stir
to blend. Add the remaining hot milk and blend well. Transfer the entire
mixture back to the saucepan over medium heat.
In a small bowl, combine the 1/4 of water with the cornstarch until smooth.
Add the cornstarch mixture to the saucepan and stir until thickened.
Add the vanilla and stir until completely blended. Pour the mixture into a bowl.
Cover with plastic wrap placed directly on the surface of the filling.
Refrigerate until cold.