Preheat the oven to 180 °C (350 °F). Grease the springform pan and line the bottom with parchment paper.
Place chopped dark chocolate and butter in a heatproof bowl set over a pan of simmering water (double-boiler). Stir until completely melted and smooth. Remove from heat and let cool slightly.
In a large bowl, whisk egg yolks together with half the sugar (100 g) and vanilla extract until pale and thick—about 3–4 minutes.
In a clean bowl, beat egg whites with a pinch of salt. When they begin to foam, gradually add the remaining sugar and continue beating to stiff, glossy peaks.
Gently fold the melted chocolate mixture into the yolk mixture until just combined.
Sift flour and cocoa powder over the chocolate batter and fold in carefully to avoid lumps.
Fold one-third of the egg whites into the batter to lighten it, then gently fold in the remaining whites until no streaks remain.
Pour the batter into the prepared pan, smooth the surface, and bake for 25–30 minutes. The cake should have a slight wobble in the center.
Remove from oven and let cool in the pan for 10 minutes, then release the springform and transfer the cake to a wire rack to cool completely.
While the cake cools, prepare the ganache: heat the cream in a small saucepan until it just begins to simmer. Pour over the chopped chocolate in a bowl. Let stand 1 minute, then stir until smooth. Stir in the butter until melted and glossy.
Allow ganache to cool and thicken slightly (5–10 minutes), then pour over the top of the cooled cake, letting it drip down the sides.
Chill the cake for at least 1 hour to set the ganache. Decorate with chocolate shavings or fresh berries if desired.
Slice and serve at room temperature. Enjoy your authentic Swiss chocolate cake!
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