In a saucepan, bring plums and water to a boil. Cover; remove from the heat and let stand 10 minutes. Drain; reserving juice.
Unroll one tube of rolls and press into a greased 13-in. x 9-in. baking pan. Spread with half of the frosting. Cover with plums. Combine 1 tablespoon butter and 1 tablespoon reserved plum juice; drizzle over plums.
Roll out remaining rolls into a 13-in. x 9-in. rectangle. Place over plums. Combine remaining frosting and butter with 1 tablespoon plum juice. Carefully spread over dough.
Bake at 375° for 25-30 minutes or until golden brown. Cool for 10 minutes. Combine confectioners' sugar with 1-2 tablespoons plum juice; drizzle over cake. (Discard remaining plum juice).