Melt the chocolate by finely chopping the semi-sweet chocolate and placing it in a microwave-safe bowl. Heat in the microwave at medium power for 30-second intervals, stirring frequently until melted and smooth, about 1-2 minutes.

Bloom the cocoa powder by bringing water to a boil and mixing it with the Dutch-processed cocoa powder until smooth.

Combine the melted chocolate with the prepared cocoa mixture and add vanilla extract.

Whip the heavy cream with sugar and salt until soft peaks form, about 3-5 minutes.

Fold in a quarter of the whipped cream into the chocolate mixture, then incorporate the remaining cream in two additions until smooth.

Chill the mousse in individual cups or a bowl covered with plastic wrap for at least 3 hours.