Combine the first 3 ingredients in a bowl; stir well, and set aside. Combine cocoa, 2/3 cup Prune Purée, water, salt, peppermint extract, and sugar in a large bowl; beat at medium speed of a mixer until smooth. Reserve 3/4 cup cocoa mixture; add remaining 1/3 cup Prune Purée and eggs to remaining cocoa mixture in bowl. Beat at medium speed until smooth. Gradually add the flour mixture, beating at low speed until smooth. Stir in boiling water.
Pour batter into a 13 x 9-inch baking pan coated with cooking spray, spreading evenly. Bake at 350° for 35 minutes or until the cake springs back when touched lightly in center. Let cool completely in pan on a wire rack.
Fold whipped topping into reserved 3/4 cup cocoa mixture; spread over cake. Cover and chill.