Preheat oven to 450°.

Line cookie sheets with parchment paper or silicone sheets.

In a medium mixing bowl, combine the flour, cocoa powder, baking soda, and salt; set aside for now.

In a large mixing bowl, cream together ½ cup shortening and sugar until light and fluffy.

Add the egg, egg yolk, and vanilla and beat for 2 minutes more.

Add ⅓ of the flour mixture and beat on low speed just until combined.

Add the buttermilk, ⅓ of the flour mixture, the hot water, and the remaining flour mixture, beating on low speed with each addition until combined.

Drop the dough onto the prepared cookie sheets using about 2 tsp. for each cookie.

Bake for about 5 minutes; let cool on the cookie sheets for several minutes and then remove them to a wire rack to cool completely.

In the meantime, make the filling.

Cream together the marshmallow crème and shortening for about 3 minutes, using an electric beater and beating until the mixture is light and fluffy.

Reduce speed to lowest setting and mix in the powdered sugar, a bit at a time; add the vanilla.

Now increase the speed to medium and beat the filling until light and fluffy, about another 3 minutes.

To make the whoopie pies, spread some filling on the bottom side of half of the cookies and then place another cookie (top side out) on the filling.

Press gently but not so hard that the filling oozes out.