Combine boiling water and cocoa. Stir until smooth. Set aside to cool.
Sift together flour, sugar, baking powder and salt in a bowl. Make a well in the center and add oil, egg YOLKS, cooled cocoa mixture, and vanilla, in that order. Beat slightly with each addition; after all are added, beat until smooth.
In a separate large bowl, beat egg WHITES with cream of tartar until very stiff peaks form.
Slowly pour the chocolate mixtre over the eggwhites in a thin stream Fold gently until well blended.
Bake in an UNgreased 10" tube pan for 55 minutes at 325°F Then increase heat to 350°F and bake for another 10 minutes.
Invert pan and cool. Ice with chocolate icing, if desired.