Preheat the oven to 350℉. Grease a baking sheet and set aside.
To make the pies, in a stand mixer using the paddle attachment, cream the brown sugar, shortening, and egg together. In a separate bowl, mix together the flour, cocoa powder, baking powder, baking soda, and salt. Slowly add the dry mixture to the creamed mixture, ensuring they are well mixed. Slowly add in the milk and vanilla until the batter is well mixed.
Place a spoonful of the batter onto the prepared baking sheet to make 12 pie halves, smoothing them to ensure each pie is as level as possible for even baking. Make sure you give enough space between each, as they will spread slightly during baking.
Bake for 12 to 15 minutes.
Remove the pies from the oven and allow them to cool completely on a wire rack.
While pies are cooling, make the filling. Using a stand mixer with the whisk attachment, combine the marshmallow fluff, powdered sugar, and shortening. Add the vanilla and mix until well combined.
To assemble the Whoopie Pies, take one pie and spread it with a generous amount of filling. Take a second pie and press together to make a sandwich. Continue until all the pies are assembled