Mix together brown sugar, oil and eggs. Add cinnamon, allspice, salt, baking powder, vanilla and baking soda. Thoroughly mix. Add pumpkin and flour, beating well. Pour into a 9 x 13 inch cake pan and bake at 350° for 30 to 40 minutes. Cool. When cake is cool, slice lengthwise. Remove top layer of cake carefully and set aside.
Cook milk and flour until thick, stirring constantly. Cool. Then add sugar, shortening and vanilla and beat until smooth and creamy. Spread the filling over the bottom layer of the cake. Then very carefully take the top layer of the cake and put on top of the filling.