Melt the Chocolate: Chop the semi-sweet chocolate into small pieces and place it in a microwave-safe bowl. Heat in the microwave at 20-30 second intervals, stirring after each until melted and smooth.

Bloom the Cocoa Powder: Boil water and transfer it into a mixing bowl. Whisk in the Dutch-processed cocoa powder until fully dissolved, then fold this cocoa mixture into the melted chocolate.

Add Vanilla Extract: Pour in the vanilla extract into the chocolate mixture and stir to incorporate evenly.

Whip the Cream: In a separate bowl, whip the cold heavy cream with sugar and a pinch of salt until soft peaks form.

Combine Ingredients: Fold in a quarter of the whipped cream into the chocolate mixture, then add the remaining whipped cream gradually, folding carefully.

Chill the Mousse: Transfer the mousse into serving cups or a large bowl, cover, and refrigerate for at least 3 hours.

Garnish and Serve: Once set, optionally top with whipped cream, chocolate shavings, or fresh berries, and serve.