In a large saucepan, whisk together milk, ¼ cup sugar, and vanilla extract. Heat over medium heat until steaming. (Do not boil.)
In a large bowl, whisk together egg yolks, cornstarch, salt, and remaining ¼ cup sugar. Gradually add warm milk mixture to egg yolk mixture, whisking constantly. Return mixture to saucepan, and cook over medium heat, whisking constantly, until thickened and boiling, 4 to 5 minutes. Strain mixture through a fine-mesh sieve into a large bowl. Stir in butter in 2 additions. Stir in pistachios. Cover with a piece of plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until completely chilled, approximately 4 hours or overnight.
Recipe by TeaTime Magazine at https://teatimemagazine.com/pistachio-eclairs/