Preheat oven to 350°.
To prepare cake, coat 2 (8-inch) round cake pans with cooking spray; dust each with 2 teaspoons flour.
Place the prunes and water in a small saucepan, and bring to a boil. Remove from heat; cover and let stand 10 minutes. Drain.
Beat brown sugar and 1/2 cup butter at medium speed of a mixer until well-blended. Add egg substitute; beat well
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine flour and next 6 ingredients (flour through salt) in a bowl. Add flour mixture to creamed mixture alternately with buttermilk, beginning and ending with flour mixture. Fold in prunes. Pour batter into prepared cake pans. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove from pans. Cool completely on a wire rack.
To prepare frosting, beat cream cheese and 3 tablespoons butter at medium speed of a mixer until smooth. Add rind, milk, and vanilla, and beat well. Gradually add powdered sugar, beating until smooth. Place 1 cake layer on a plate; spread with half of frosting. Top with remaining cake layer. Spread remaining frosting over top of cake. (Do not spread frosting on sides of cake.)