Accompaniment: fat-free frozen yogurt or raspberry sauce and powdered sugar.
Preheat the oven 350 degrees F. Spray a 9-inch cake pan with cooking spray and dust with additional cocoa powder, set aside.
In a medium bowl, combine the flours, sugar, cocoa, baking powder, baking soda, and salt; set aside. In another bowl, combine the prune puree, egg whites, vanilla, and coffee. Mix the wet ingredients into the dry ingredients and pour into the prepared cake pan.
Bake for 20 to 25 minutes or until a toothpick inserted in the middle of the cake comes out clean. Cool the cake in the pan on a wire rack for 10 minutes. Run a knife around the edge of the cake and invert onto a wire rack to cool completely. Cut into 14 wedges to serve.