Open the coconut milk, and leave the can (or transfer to a bowl) uncovered in the fridge overnight, unless it’s already thick like mousse. Don’t shake the can before opening. It should get very, very thick. (If it doesn’t, you’ve gotten a bad can that won’t work for the recipe. I highly recommend Thai Kitchen Organic, because I’ve had the most success with that brand. If you want a no-fail option, buy coconut cream instead, at Trader Joes or an Asian market.) Once thick, transfer to a bowl (you can opt to leave out the watery bit at the bottom of the can, if you want it even thicker) and whip in your cocoa, vanilla, and sweetener with a fork, or even beaters if you want to be fancy. (For the photos on this page, I didn’t use beaters. But I did pipe the mousse out using an icing tip.) Stored uncovered in the fridge, the mixture gets even thicker.