Preheat oven to 350°F (175°C). Line the bottom of a 9-inch springform pan with parchment paper.
Melt the dark chocolate and butter together using a double boiler or microwave in short intervals, stirring until smooth. Let cool slightly.
In a mixing bowl, whisk egg yolks with half of the sugar until pale and creamy. Stir in vanilla extract.
Gradually mix the melted chocolate mixture into the egg yolk mixture until fully combined.
In a separate bowl, beat egg whites with salt until soft peaks form. Slowly add the remaining sugar and continue beating until stiff, glossy peaks form.
In another bowl, whip the cold heavy cream until soft peaks form.
Gently fold the whipped cream into the chocolate mixture, then carefully fold in the beaten egg whites in two additions, keeping the mixture light and airy.
Pour the mousse batter into the prepared pan and smooth the top.
Bake for 25–30 minutes, until the edges are set but the center is slightly soft.
Remove from oven and let cool completely, then refrigerate for at least 4 hours or overnight for best texture.
Before serving, dust lightly with cocoa powder and slice gently.