Put the boiling water and cocoa powder in a blender. Blend on high speed for about 30 seconds, or until the mixture is relatively smooth. Scrape down the sides of the blender. Be careful not to let steam build up.
Add the dates, brown rice syrup, and salt to the blender. Blend until smooth, stopping occasionally to scrape down the sides of the blender with a spatula to make sure all the ingredients are incorporated. Add the vanilla and blend until combined.
Transfer the mixture to an airtight container. Let chill completely, for at least 3 hours, until it becomes firm and spreadable.
Spread the frosting onto cooled cupcakes with the back of a spoon or an offset spatula. You can also pipe the frosting with a pastry bag fitted with a wide metal tip.
Isa Chandra Moskowitz has been cooking up a vegan storm for the past two decades. She is the author of seven cookbooks, including Veganomicon and Appetite for Reduction. A Brooklyn native, she now lives in Omaha, Nebraska, where she spreads vegan cooking love and does feral cat rescue work.