Mix dry ingredients. Add all other ingredients. Then mix. Pour into a greased and floured jelly roll pan. Bake at 350 degrees for 20 to 30 minutes. Touch middle to detect doneness. Cool completely, then ice with powdered sugar frosting.
Frosting: Mix cream cheese and butter. Add vanilla. Add powdered sugar, a cup at a time. Spread frosting over bars. Sprinkle with walnuts (if desired). Store covered in refrigerator.