Add buttermilk, sugar, cocoa powder, salt, and vanilla into a blender. Pulse a few times to combine the ingredients, then blend on medium speed for 1 minute. Scrape down the sides.
Add 1 cup of water to the pressure cooker pot and place a trivet inside. Place three ramekins on the trivet, top with another trivet, follow with three more ramekins.
Place ramekins on a cooling rack and remove foil and coffee filters (you can reserve the foil for covering the ramekins and storing them in the refrigerator later).