Preheat the oven to 325 degrees F. Prepare a 9-inch by 13-inch rectangular cake pans by spraying the bottom and sides with nonstick spray. Set aside.
In a medium size mixing bowl, whisk together the dry ingredients. Set aside.
In another medium-size mixing bowl, whisk together the eggs, buttermilk, water, oil and vanill.
Add wet ingredients to the dry ingredients, and mix on low speed or with a whisk until combined. Batter will be thin. Scrape sides and mix for another 30 seconds. Pour the batter into the cake pan.
Bake for 30 to 35 minutes (you may need more time depending on your oven), until toothpick comes out with just a few moist crumbs on it. Cool completely.
Keep the cake in the baking dish for assembly.