In a bowl, cream butter and sugar. Combine flour, baking powder and salt; add to creamed mixture alternately with milk. Stir in cranberries. Pour into a greased 8-in. square baking pan. Bake at 350° for 40 minutes or until lightly browned. Meanwhile, for sauce, combine sugar, flour and salt in a saucepan. Add water and stir until smooth. Cook and stir until mixture comes to a boil. Reduce heat; cook and stir 3 minutes more. Remove from the heat; stir in butter and vanilla. Cool slightly; serve over warm pudding. Refrigerate leftover sauce.