Preheat the oven to 180°C (350°F). Grease a 9-inch round cake pan and line the bottom with parchment paper.
Melt the chocolate and butter in a heatproof bowl set over a pot of simmering water, stirring until smooth. Let it cool slightly.
In a separate bowl, whisk the egg yolks with the sugar and vanilla until pale and thick. Stir in the melted chocolate mixture.
In a clean bowl, whisk the egg whites and a pinch of salt until stiff peaks form. Gently fold the egg whites into the chocolate mixture in three additions.
Pour the batter into the prepared pan and smooth the top. Bake for 25-30 minutes until the edges are set and the center is slightly soft.
Allow the torte to cool in the pan for 10 minutes, then gently remove from the pan and cool completely on a wire rack.
Serve the torte with a dusting of cocoa powder or a dollop of whipped cream if desired.