Preheat the oven to 325 degrees F. Prepare 9-inch by 13-inch rectangular cake pans by spraying the bottom and sides with nonstick spray, lining the bottom with parchment paper, and spray the parchment paper. Set aside.
In a bowl of electric mixer, combine all the dry ingredients.
Combine eggs, buttermilk, water, oil and vanilla in a measuring cup and lightly beat with a fork.
Add wet ingredients to the dry ingredients, and mix on low speed until combined. Batter will be thin. Scrape sides and mix for another 30 seconds. Pour the batter into the cake pan.
Bake for 25 to 30 minutes (you may need more time depending on your oven), until toothpick comes out with just a few moist crumbs on it. Cool completely before frosting.