Adjust rack in lower third of oven. Preheat oven to 350 degrees. Grease and flour two 20 cm round cake pans, and insert parchment paper or waxed paper to line the bottoms. Sift the flour, baking soda, and salt onto a sheet of waxed paper; set aside. Place the cocoa in a 1 litre mixing bowl. Add the half cup lukewarm water, and whisk to combine; set aside to cool. Pour the buttermilk, the half cup water, and the vanilla into a liquid cup measure. Crack the eggs into a small bowl, and whisk together to combine the yolks and whites. Place the butter in the bowl of a heavy-duty mixer, preferably fitted with a flat beater. Cream the butter on medium speed until the butter is smooth and lighter in color, about 30 to 45 seconds. Reduce the speed to low, add the sugars in a steady stream. When all the sugar is added, stop the machine, and scrape the mixture clinging to the side of the bowl into the center. The mixture will appear sandy. Increase the speed to medium again, and cream until the mixture is light in color, is fluffy in texture, and appears as one mass, about 5 to 7 minutes. With the mixer still on medium speed, pour the eggs slowly at first. Continue to cream, scraping the sides of the bowl at least once, until the mixture appears fluffy and velvety. Stop the machine and spoon in the cooled cocoa mixture, resume at medium speed and mix just until incorporated. Using a rubber spatula, stir in one-fourth of the flour mixture. Then one-third of the buttermilk mixture, stirring to blend together. Repeat this procedure, alternating dry and liquid. With each addition, scrape thesides of the bowl, and continue mixing until smooth, never adding liquid if any flour is visible. Pour the batter into the pans and spread it level. Bake for 25 minutes, or until the baked surface springs back slightly when touched lightly in the center and the sides contract from the pan. Place the cake pans on a rack to cool for 5 to 10 minutes. With mitts, tilt and rotate pans gently tapping them on the counter to see if the cake releases from sides. Two 20 cm layers. ----