Preheat the oven to 325 degrees. Have an ungreased 10-inch tube pan ready.
Combine the cocoa and boiling water.
Beat the egg whites with the cream of tartar until very stiff peaks form.
Sift together the cake flour, sugar, baking soda and salt into a mixing bowl. Make a well in the center. Add oil, egg yolks, cocoa mixture and vanilla; beat well. Fold in the egg whites; blend well. Pour into the ungreased tube pan; cut through the batter with a spatula. Bake at 325 degrees for 55 minutes. After 55 minutes, increase the temperature to 350 degrees and bake for 10 minutes longer, or until the cake tests done. Invert the pan to completely cool the cake.
Source: “Homemade Ice Cream and Cake”, Farm Journal, Doubleday and Company, 1972