In a mixing bowl, beat pumpkin, eggs, oil and vanilla until well mixed. Combine dry ingredients; stir into pumpkin mixture and mix well. Pour into a greased 15x10x1 inch baking pan. Bake at 350° 20 to 25 minutes or until brownies test done with a toothpick. Cool. In a small mixing bowl, beat the butter, cream cheese, vanilla, milk and salt until smooth. Add confectioners' sugar, mix well. Frost brownies. Store in the refrigerator.