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, sugar, baking soda 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 stiff peaks form. Gradually fold into egg yolk mixture.
Gently spoon batter into an ungreased 10-in. tube pan. Cut through the batter with a knife to remove air pockets. Bake on lowest rack at 325° for 60-65 minutes or until top springs back when lightly touched. Immediately invert pan; cool completely. Run a knife around sides and center tube of pan. Invert cake onto a serving plate.
For icing, melt butter in a saucepan. Remove from the heat; stir in the confectioners’ sugar, chocolate, vanilla and water. Drizzle over cake. Sprinkle with nuts if desired.Yield: 16-20 servings.