Preheat oven to 350 degrees. Coat a 9 inch square pan with cooking spray. In a mixer bowl, combine water, prune puree, egg whites and vanilla. Beat thoroughly to blend. Combine flour, sugar, cocoa, baking powder, baking soda and salt. Beat dry ingredients into liquid mixture. Spread batter into prepared pan. Bake about 30 minutes, or until cake tests done. Cool on rack in pan. Serve dusted with powdered sugar or with the following icing.
NOTE: To prepare prune puree, simmer 1/2 c prunes in water to cover for about 10 minutes. Let stand until cool. Drain well. Force the prunes through a sieve, leaving behind the skin particles and seeds.
ICING: Combine 2 1/2 c powdered sugar, 1/4 c cocoa and 1/4 c low fat milk. Spread on cooled cake.