Let's cook.
In a medium saucepan, combine 3 and 1/2 cups of whole milk and the granulated sugar. Heat over medium heat, stirring occasionally, until the sugar dissolves and the milk is warm but not boiling.
In a small bowl, whisk together the remaining 1/2 cup of whole milk with the cornstarch until a smooth slurry forms.
Gradually whisk the cornstarch slurry into the warm milk mixture in the saucepan. Continue to cook over medium heat, stirring constantly, for 8-10 minutes, or until the mixture thickens to a pudding-like consistency.
Remove the saucepan from the heat and stir in the rosewater.
Pour the mahalabia into individual serving bowls or ramekins. Let it 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 chopped dates and chopped pistachios.
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