Preheat oven to 325 degrees. Spray the bottom of the three 8-inch or four 6-inch round cake pans with cooking spray and line bottoms with parchment. Set aside.
In a large mixing bowl combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In a medium bowl or measuring cup, combine the eggs, buttermilk, water, oil and vanilla.
With the mixer on low speed, gradually add the wet ingredients to the dry ingredients and mix until smooth, about 30 seconds. Scrape down the sides of the bowl and mix again for another 10 to 20 seconds. Divide batter equally among the three cake pans (about 16 ounces of batter in each 8-inch pan or 12 ounces in each 6-inch pan).
Bake the layers for about 25 to 30 minutes (time may vary depending on your oven) or until a wooden pick inserted into the center of the cake comes out with only a few moist crumbs on it. Let the cakes cool in pans on a wire rack for 10 minutes, and then turn onto a rack to cool completely. When completely cooled, wrap the layers in plastic wrap and chill for at least an hour in the freezer. Cakes can be make a week ahead of time and wrapped in plastic wrap. If you'd like to store the cake layers longer than a week, wrap in tin foil, as well, and then place in a zip lock bag.