In a mixing bowl, cream together sugar and vegetable shortening until light and fluffy.
Add eggs one at a time, mixing well after each addition.
In a separate bowl, sift together flour, cocoa powder, and salt.
Dissolve baking soda in hot water and add to the sour milk.(Milk can be soured with 1 TBS vinegar or fresh lemon juice per cup milk. Let stand 5 minutes.)
Add vanilla extract to the wet ingredients.
Gradually add the dry ingredients to the wet mixture, mixing until well combined.
Drop spoonfuls of dough onto a baking sheet and bake at 350°F for 10-12 minutes.
For the cream filling:
See Whoopie Cookie Filling Recipe here on this site.
Spread a generous amount of filling onto the bottom side of one cookie and sandwich with another cookie.
Repeat with remaining cookies and filling to make chocolate whoopie pies.