Cook prunes according to package directions; drain, reserving liquid. Chop prunes; set aside. Add enough milk to prune liquid to equal 1 cup; set aside.
Beat butter at medium speed with an electric mixer until creamy; gradually add brown sugar, beating well. Add egg, beating until blended.
Combine flour and next 4 ingredients; add to butter mixture alternately with milk mixture, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in chopped prunes and vanilla. Spoon batter into greased muffin pans, filling two-thirds full.
Bake at 400° for 22 to 25 minutes. Remove from pans immediately.