Combine the milk, cocoa powder, dates, vanilla and a pinch of salt in a medium saucepan. Heat the mixture over medium heat, stirring frequently, until hot and steaming. Continue to cook, without letting the mixture boil, giving the dates time to soften, another 5 minutes.
Let cool for 5 minutes or so, then transfer the mixture to a blender and being careful of the hot liquid, puree until smooth. You can strain the mixture to catch any small chunks of dates that weren't blended.
Pour into mugs and top each with whipped cream and a sprinkle of cinnamon, if using.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.
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