Begin by placing the chopped dark chocolate and water in a heatproof bowl. Set the bowl over a pot of gently simmering water, making sure the bowl doesn’t touch the water. Stir frequently until the chocolate is fully melted and smooth. Remove from heat and let it cool slightly while you prepare the other components.
In a clean, dry bowl, add the egg whites and a pinch of salt. Using an electric mixer, beat the whites on medium speed until they become foamy. Gradually add the granulated sugar, continuing to beat until stiff peaks form. Set aside.
In another bowl, whisk the egg yolks with the vanilla extract until smooth. Slowly pour the melted chocolate into the yolks, stirring constantly to combine.
In a chilled bowl, whip the heavy cream until soft peaks form. The cream should be light and airy but not over-whipped.
Gently fold the whipped cream into the chocolate mixture using a spatula. Then fold in the beaten egg whites in two batches, maintaining as much air as possible for a light mousse texture.
Spoon the mousse into individual cups or ramekins. Cover and refrigerate for at least 2 hours, or until set. Just before serving, garnish with fresh berries.