Instructions

  1. Preheat oven to 350 degrees.
  2. Sift together the flours, cocoa, sugar and baking soda into a large mixing bowl.
  3. In a small bowl combine the buttermilk, water (oil, if using) and vanilla extract.
  4. Combine the buttermilk mixture with the dry ingredients and pour into a 9" by 13" pan prepared with a light dose of cooking spray. Bake for 30-33 minutes or until a toothpick in the center comes out clean.
  5. Cool the cake to room temperature.
  6. For the frosting, combine the cocoa (if using) with the yogurt, then fold in the whipped topping.
  7. Top the cake with the frosting and chill for 3 hours to overnight.