Start by roughly chopping semi-sweet chocolate into small pieces and place in a microwave-safe bowl. Heat in 20-30 second intervals, stirring until melted and smooth.

In a saucepan, bring water to a boil. Whisk in cocoa powder until smooth, then remove from heat and let it cool.

Gradually add the cooled cocoa mixture to melted chocolate while whisking continuously to create a smooth base. Stir in vanilla extract.

In a mixing bowl, combine cold heavy cream, sugar, and salt. Whip until soft peaks form.

Gently fold in one-fourth of the whipped cream into the chocolate mixture to lighten it.

Fold in the remaining whipped cream in two stages, being careful not to deflate the mixture.

Pour the mousse into serving cups and refrigerate for at least 3 hours to set.

Serve chilled, garnished with whipped cream, chocolate shavings, or berries as desired.