Let's cook.
In a medium saucepan, whisk together 1/2 cup of the whole milk with the cornstarch until smooth and no lumps remain. This creates a slurry.
Add the remaining 2 1/2 cups of whole milk and the granulated sugar to the saucepan. Whisk well to combine.
Place the saucepan over medium heat, stirring constantly with a whisk or wooden spoon. Continue to cook for 10-15 minutes, or until the mixture thickens to a pudding-like consistency and coats the back of a spoon.
Remove the saucepan from the heat. Stir in the rosewater and cardamom powder until fully incorporated.
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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