Let eggs stand at room temperature for 30 minutes. In a bowl, combine cocoa and water until smooth; cool for 20 minutes. In a large bowl, combine flour, 200g sugar, and salt. In a bowl, whisk the egg yolks, oil and vanilla; add to dry ingredients along with the cocoa mixture. Beat until well blended.
In another large bowl and with clean beaters, beat egg whites and cream of tartar on high speed until soft peaks form. With the motor still running, add one tablespoon of the remaining sugar (200g) in one at a time and beat until stiff peaks form. Gradually fold into egg yolk mixture.
Gently spoon batter into an un-greased 24cm tube/chiffon pan. Cut through the batter with a knife to remove air pockets. Bake on lowest rack at 160° for 60-65 minutes or until top springs back when lightly touched. Immediately invert pan; cool completely. Run a knife around sides and centre tube of pan. Invert cake onto a serving plate.
For icing, melt butter in a saucepan. Remove from the heat; stir in the icing sugar, chocolate, vanilla and water. Drizzle over cake. Decorate with nuts, flowers or whatever your heart desires.