Preheat oven to 350°F (175°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 ½ cup shortening and white sugar. Beat in the eggs one at a time, then stir in the vanilla extract and cocoa mixture.

Combine 2 ⅔ 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. Cool completely.

To make the filling: In a small saucepan, stir together the remaining flour and a 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 ¾ cup shortening and confectioners' sugar until light and fluffy. Gradually add the vanilla and the milk mixture, beating until fluffy.