Preheat the oven to 350°F (175°C). Prepare your cake pan by greasing it with butter and lining the bottom with parchment paper. Dust the sides with cocoa powder.

In a heatproof bowl, combine the dark chocolate and butter. Melt them together over a pot of simmering water or in the microwave. Stir until smooth and let it cool slightly.

In another bowl, whisk the egg yolks and granulated sugar until pale and creamy. Add the vanilla extract and mix well.

Slowly add the melted chocolate and butter to the egg yolk mixture. Stir until fully combined.

In a separate bowl, beat the egg whites and a pinch of salt until soft peaks form. Gradually add the sugar and continue to whip until stiff peaks form.

Gently fold one-third of the beaten egg whites into the chocolate mixture to loosen it, then carefully fold in the remaining egg whites.

Pour the batter into the prepared cake pan and smooth the top. Bake for 25-30 minutes until slightly cracked on top but still soft.

Remove from the oven and let the cake cool in the pan for 10 minutes. Carefully invert onto a plate and remove the parchment paper.

Dust the top of the cooled cake with cocoa powder before serving.