Preheat oven to 350 degrees F.
Mix cocoa and water in a bowl and set aside to cool.
Grease and flour three 9x3 1/2 inch cake pans.
Mix flour, soda, baking powder, and salt in a bowl.
Cream butter. Stir in sugar. Add eggs one at a time, beating mixture after each addition.
Stir in vanilla.
Add alternately 1/3 of flour mixture and 1/2 of cocoa mixture stiring after each addition.
Divide into the prepared cake pans and bake for 30 minutes.
Recently, I had a couple of leftover egg whites. I added a little extra flour, seperated the eggs as I added them to the butter/sugar mixture then at the end, beat the egg white to soft peaks and folded them into the batter. Worked well.