Beat egg whites with a little salt until soft peaks form. Add the sugar gradually, beating well after each addition, until sugar has dissolved and mixture is thick and glossy. Fold one-third of egg white mixture into chocolate mixture until just combined. Repeat with remainder, in two batches. Spoon mixture into prepared tin. Bake for 1 hour. Remove and cool at room temperature for 30 minutes. Gently run a knife around the edges and carefully remove cake from pan. Dust cake with cocoa or icing sugar and cut into slices.Serve with double cream and crushed raspberries if desired.