Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a separate large bowl, cream the softened butter and brown sugar until light and fluffy.
Add the egg and vanilla extract to the butter mixture, mixing until well combined.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined.
Scoop out tablespoons of batter onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes, or until the cakes spring back when lightly touched.
Allow the cakes to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
In a bowl, combine the powdered sugar, marshmallow fluff, and vegetable shortening to make the filling.
Spread a generous amount of filling on the flat side of half the cooled cakes, then sandwich with another cake.
Serve the chocolate whoopie pies immediately or store them in an airtight container.