Make sure to use a quality brand of coconut milk because if the cream part is too liquidy, it will not whip up properly. I like Thai Kitchen found in the Asian section of the grocery store. Place the can in the fridge overnight, this will help it to thicken and separate the cream at the top of the can, which you will only be using. When you remove the can, do NOT shake it. All the cream is sitting at the top of the can.
When you remove the lid, the cream should be thick and somewhat hardened. If it is too liquidy, then the recipe won't work well. Just carefully scoop out all the white cream into a bowl and once you get to the watery part, stop scooping. (save the extra liquid for a smoothie) Depending on your can (they vary) and brand, you may need to adjust the sweetner or cocoa depending on how much cream you have. You should get about 3/4 cup of cream.
With an electric mixer, begin beating on low speed and gradually progress to high speed. After you've whipped the cream for about 5-8 minutes and it has begun to thicken, add the cocoa, sugar and vanilla. Whip again for a minute. Taste and adjust any sweetener, if desired.
Whip for a few more minutes until light and fluffy.
Place in the fridge to chill and set, at least an hour. The longer it sits, the more stability it will have. Serve with fruit, over a shake, on a cake.