Preheat oven to 350°F.
Lightly coat an 8 x 8-inch pan with cooking spray.
In a large bowl combine wheat flour, all-purpose flour, sugar, cocoa powder, baking powder.
In a medium bowl beat egg whites until foamy; add baby-food prunes, buttermilk, and vanilla extract. Pour into the flour mixture and blend well. Stir in mini chocolate chips.
Spread batter in pan and sprinkle with walnuts if desired. Bake 25 minutes or until toothpick inserted near the center comes our clean.
Let cool for 30 minutes in the pan before cutting.