Preheat the oven to 165 degrees C. Grease and flour a 23cm springform cake tin.
Set aside 3 tablespoons of the sugar. In a heavy based frypan melt the chocolate, butter and remaining sugar. Remove the frypan from heat. Add the vanilla and allow to cool slightly.
Beat the egg yolks into the chocolate mixture, one at a time, beating well after each addition. Stir in the flour.
In the bowl of an electric mixer, extremely clean and free of grease, whip the egg whites until soft, then add cream of tartar and salt, then beat to stiff peaks. Add the reserved sugar and continue beating until glossy. Beat 1/3 of the egg whites into the chocolate mixture, then carefully fold in the remaining whites. Carefully pour batter into the prepared cake tin then tap it gently to release air bubbles.
Bake for about 45 minutes to 1 hour until well risen and a skewer inserted into the centre of the cake comes out clean. Remove from oven, remove the sides of the spring form tin. Cool completely then remove the base. Be careful when removing the base, this cake is very delicate.