Let's cook.
In a medium saucepan, whisk together 2 1/2 cups of whole milk, granulated sugar, and salt over medium heat until the sugar dissolves and the mixture is warm, but not boiling.
In a small bowl, whisk the remaining 1/2 cup of cold milk with the cornstarch until completely smooth and no lumps remain.
Gradually pour the cornstarch mixture into the warm milk in the saucepan, whisking continuously to prevent lumps.
Continue to cook over medium heat, stirring constantly with a whisk or wooden spoon, until the pudding thickens to a custard-like consistency, about 8-10 minutes. It should coat the back of a spoon.
Remove the saucepan from the heat and stir in the rosewater.
Pour the pudding into individual serving dishes or a single large bowl. Cover the surface directly with plastic wrap to prevent a skin from forming.
Refrigerate for at least 2 hours, or until thoroughly chilled and set.
Before serving, garnish generously with chopped pistachios.
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