Start by roughly chopping the semi-sweet chocolate and placing it in a microwave-safe bowl. Microwave at 20-30 second intervals, stirring frequently until the chocolate is completely smooth and glossy. This usually takes about 1-2 minutes. Set the melted chocolate aside to cool slightly while you prepare the cocoa mixture.

In a small saucepan, bring 1 cup of water to a boil, then turn off the heat. Add in the Dutch-processed cocoa powder, stirring vigorously until the cocoa is fully dissolved and the mixture is smooth.

Pour the bloomed cocoa mixture into the cooled melted chocolate. Add the vanilla extract and whisk together until smooth and well blended.

In a large mixing bowl, whip the cold heavy cream, sugar, and a pinch of salt together using a hand mixer until soft peaks form, about 2-3 minutes.

Take about one-quarter of the whipped cream and gently fold it into the chocolate mixture using a spatula.

Gradually fold in the remaining whipped cream in two additions. Use a gentle folding technique to maintain lightness.

Pour the chocolate mousse mixture into serving cups or a large bowl. Cover with plastic wrap and refrigerate for at least 3 hours.

Once set, remove from the refrigerator, garnish with whipped cream, chocolate shavings, or fresh fruit, and serve chilled.