Preheat the oven to 375 degrees F and grease a 9-inch round cake pan.
Melt the coconut water, chocolate, and butter over a double boiler or in the microwave, and let cool.
In a medium bowl, combine the egg yolks and sugar, and whisk to a ribbon-like consistency. Add the chocolate mixture to the yolk mixture with a rubber spatula.
Whip the egg whites and salt to a soft peak. Gently fold the egg white into the chocolate mixture and pour the batter into the prepared cake pan.
Bake until the top of the cake loses its shine and cracks appear, about 40 minutes. Remove from the oven and let cool in the cake pan. To remove the cake from the pan, invert onto a serving plate.