Let's cook.
In a medium saucepan, whisk together 2 1/2 cups of whole milk and the granulated sugar over medium heat until the sugar dissolves and the milk is warm, but not boiling.
In a small bowl, whisk the remaining 1/2 cup of cold milk with the cornstarch until smooth and no lumps remain. This is your cornstarch slurry.
Gradually pour the cornstarch slurry into the warm milk mixture in the saucepan, whisking continuously to prevent lumps.
Continue to cook over medium heat, stirring constantly with a whisk or wooden spoon, for 8-10 minutes, or until the mixture thickens to a pudding-like consistency. It should coat the back of a spoon.
Remove the saucepan from the heat and stir in the ground cardamom and rosewater until well combined.
Pour the mahalabia into individual serving bowls or ramekins. Allow to cool at room temperature for 15 minutes.
Cover the bowls with plastic wrap and refrigerate for at least 2 hours, or until thoroughly chilled and set.
Before serving, garnish each mahalabia with a generous sprinkle of finely chopped pistachios.
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