Preheat oven to 375 degrees. Line two baking sheets with parchment paper and set aside.
In a large bowl, whisk together flour, cocoa, baking soda and salt very well. Set aside.
In the bowl of your stand mixer (or in another large bowl using a hand mixer), cream butter, shortening and brown sugar together until completely smooth and no lumps remain. Beat in egg and vanilla until thoroughly combined. Turn mixer on low speed and alternate adding the flour mixture and milk and mix until batter is smooth, without over-mixing.
Drop one rounded tablespoon for each whoopie pie onto prepared baking sheets 2 inches apart. Using a cookie scoop or ice cream scoop is extremely helpful for this step. Bake for 8-10 minutes or until centers are set. You absolutely do not want to over-bake these or they will come out dry.
Allow them to cool for a few minutes on the baking sheets until they are cool enough to handle before transferring to a wire rack to finish cooling completely.
While the whoopie pie cakes are cooling, prepare the filling. In a large bowl, beat butter, marshmallow creme and salt until smooth with no lumps. Gradually add confectioner's sugar and beat filling until completely smooth and creamy. If you feel it needs to be thinner, add a little milk or cream. If it needs to be thicker, add some more confectioner's sugar. Once the cakes are completely cool, pipe or spread a thick layer of filling onto one cake and sandwich another on top. Repeat the process until they are all filled.
Chocolate Marshmallow Whoopie Pies will stay soft and fresh for up to 3 days stored in an airtight container at room temperature.