Preheat the oven to 180C. Grease and line a 7-inch spring-form cake tin/cake tin with removable bottom.
Melt the chocolate and butter in a double boiler or in a heatproof bowl set over a pan of simmering water. remove from the heat and allow to cool for a few minutes.
Whisk the egg yolks and sugar until pale, thick and mousse-like, then whisk in the chocolate mixture.
Whisk the egg whites separately in a clean, grease-free bowl until soft peaks form. add a third to the chocolate mixture and fold in, using either a large metal spoon or spatula. add the remaining whisked egg whites and fold in carefully.
immediately pour the mixture into the prepared tin and bake for 40 to 45 minutes. turn off the oven. Cover the cake with a damp tea-towel and leave to cool in the oven; do not be shocked by the sinking of the centre. It will sink as it cools.
Chocolate Ganache
Combine the chocolate, cream, butter and salt in a bowl and melt them in a double boiler or set over a pan of simmering water.
Assembly
Remove the cake out of the tin and peel off the lining paper. Place it on a serving plate then ladle the ganache over the cake, making some of the ganache to drizzle down the sides as well.
Let the ganache set slightly, decorate with chocolate curls on top of the cake to serve.