First layer: Melt butter, mix well with flour. Pour into 9x13 pan. Bake at 350 degrees for 20 min. and cool.
Second layer: Mix cream cheese, powdered sugar and milk together until smooth. Pour over cooled crust.
Third layer: Bring sugar, water, blackberries and cornstarch to a boil. Cook 3 min. Let cool. Put on top of second layer; spread evenly. Top with whipped cream and refrigerate.