Combine chocolate, water and butter in a microwave safe bowl. Microwave on medium heat for 30 seconds, stir, then microwave for 30 seconds again. Stir constantly until all chocolate is melted and the mixture is smooth. Set aside to cool for 10 minutes.

While the chocolate is cooling, take one can of coconut milk out of the fridge. Open the can and discard the liquid, then scoop the firm coconut meat into the bowl of your electric mixer. Beat on medium-high speed until completely whipped and thick. Set the bowl in the fridge.

Add egg yolks, sugar and remaining water to a saucepan over medium heat. Whisk constantly until sugar is dissolved, mixture is opaque and light yellow and has thickened and can coat the back of a spoon. Remove from heat and stir in melted chocolate mixture. Place saucepan in a bowl of ice water and whisk constantly until chocolate is cooled.

Remove whipped cream from the fridge and fold it into the chocolate mixture. Once whipped cream is completely folded in, evenly pour mousse into either two or four glasses. Refrigerate for 4 hours.

A few minutes before serving, remove the other can of coconut milk from the fridge and whip the cream just as you did earlier. Once whipped, add in powdered sugar and beat just until the sugar is incorporated. Top mousse with the whipped cream and extra chocolate shavings if desired.