Pistachio Pudding

Heat the milk and ¼ cup sugar in a medium saucepan over medium heat. Stir until sugar is dissolved and bring it just to a boil. Remove from heat, add the chopped pistachios and stir with a spoon until combined. Cover the saucepan with a lid and let steep for one hour.
Set a strainer over a medium bowl. Pour the steeped mixture through the strainer and use a spoon to press as much milk out of the pistachios as possible. Discard the pistachios or reserve for another use.
Pour the strained milk back into the saucepan and set over medium heat. Meanwhile, whisk together the egg, the remaining ¼ cup sugar, cornstarch, and salt (if using) in a bowl. When the milk comes just to a boil, slowly pour it into the egg mixture, while whisking continuously.
Pour the mixture back in to the saucepan and cook over medium heat until thickened, while stirring with a rubber spatula continuously to make sure the pudding doesn't burn. Let it boil for one minute, while still stirring, then remove from heat. Stir in the vanilla and butter.
Pour into a bowl and cover with plastic wrap, placing it directly onto the surface of the pudding to prevent a skin from forming. Chill for one hour or overnight.
Serve with whipped cream and chopped pistachios.