Sift the cocoa powder into a large mixing bowl to remove any clumps. Whisk in the granulated sugar until well combined.

Whisk in the melted coconut oil and the egg. While whisking continuously, slowly pour in the hot water. Take your time to avoid scrambling the egg.

In a saucepan, heat ⅔ cup (158 ml) of heavy cream over medium-low heat until it starts to gently simmer around the edges. Slowly stream the hot cream into the cocoa mixture, whisking as you go. Then stir in the vanilla extract and salt.

With a stand mixer with the whisk attachment or hand mixer, whip the remaining cold heavy cream in a separate bowl on medium-high speed until stiff peaks form.

Gently fold in one-third of the whipped cream with a rubber spatula. Then fold in the remaining whipped cream in two additions, being careful not to deflate the mixture.

Spoon the mousse into individual serving jars or glasses, or leave it in the bowl. Cover with plastic wrap and chill for at least 4 hours or overnight until fully set.