Preheat your oven to 180°C Fan (355°F Fan) and butter a 9-inch springform tin, lining the bottom with baking paper.

Chop the dark chocolate and combine it with unsalted butter in a microwave-safe bowl. Heat in the microwave until smooth.

Sift cocoa powder and salt into the chocolate mixture and stir until combined.

Whisk egg yolks with half of the caster sugar until light and creamy.

In a clean bowl, whisk egg whites until soft peaks form. Gradually add remaining sugar and whisk until glossy and stiff peaks form.

Gently fold the chocolate mixture into the whipped egg yolks, then gently fold in the whipped egg whites.

Pour two-thirds of the batter into the prepared tin and bake for about 25 minutes.

Let the cake cool in the tin for 10 minutes, spread reserved batter on top, and return to oven for an additional 10-15 minutes.

Allow to cool for 10 minutes before slicing. Serve warm.