Preheat oven to 350 degrees F (175 degrees C). Grease cookie sheets. In a small bowl, stir together the cocoa and boiling water. Let cool for 5 minutes.
In a medium bowl, cream together 1/2 cup shortening and white sugar. Beat in the eggs one at a time then stir in the vanilla extract and cocoa mixture. Combine 2 2/3 cups flour with baking powder, baking soda and salt; stir into the sugar mixture alternately with the buttermilk. Drop by rounded tablespoons 2 inches apart onto the prepared cookie sheets.
Bake for 10 to 12 minutes in the preheated oven, until firm. To make the filling: In a small saucepan, stir together remaining flour and pinch of salt. Gradually whisk in the milk. Cook over medium heat, stirring constantly until thick, about 5 to 7 minutes. Remove from heat, cover and refrigerate. In a medium bowl, cream together 3/4 cup of shortening and confectioners' sugar until light and fluffy. Gradually add the vanilla and the milk mixture and beat until fluffy. Sandwich filling between cookies.