Stir together the cocoa and boiling water until smooth. Set it aside to cool.
Prepare the eggs:
Place the egg whites only in the bowl of a stand mixer. (Keep the egg yolks to add later.) Add the cream of tartar. Beat on high speed until the egg whites are stiff and can hold peaks.
Make the cake:
Blend together the flour, sugar, salt, baking powder and baking soda.
Make a well in the flour mixture and add the oil, egg yolks, cocoa mixture and vanilla. Beat until the mixture is smooth and creamy.
Using a rubber bowl scraper, add the whipped egg whites and fold together very gently until it is just blended.
Pour the batter into an ungreased 10" tube pan. Bake at 325º Fahrenheit for 50-55 minutes or until the top springs back when you touch it.
Immediately turn the cake upside down, resting the edges of the cake pan on 3 glasses of the same height.
Allow the cake to completely cool upside down.
Once the cake is cool, run a knife along the inside of the pan to loosen the cake from the sides.
Turn the cake out onto a cake plate to serve.
Top with whipped cream and dust with cocoa powder if desired.
Store leftover cake in an airtight container.
The calories shown are based on the cake being cut into 12 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**