Make the whoopie pies:

1. Preheat the oven to 350°F. Prepare 3 large baking sheets with parchment paper or a silicone baking mat (or prepare just one if working in batches).
2. In a large mixing bowl, add the white sugar, sour cream, vegetable oil, egg, and vanilla. Whisk until very smooth.
3. In a separate large mixing bowl, add flour, sifted cocoa powder, baking powder, baking soda, and salt. Whisk until well combined.
4. Using a spatula, fold the dry ingredients into the wet ingredients. Mix only until everything is just incorporated and smooth, but ensuring not to overmix.
5. Using a small cookie scoop or a piping bag, scoop or pipe little mounds of batter (about one tablespoonful) onto the prepared baking sheets, ensuring there is about 2 inches of space between each. You should fit about 12 cookies per sheet.
6. Place the baking sheets into the oven and bake for 10-11 minutes. When done, the cookies should have a springy top, and a toothpick should come out clean. Allow to cool completely before filling.

Make the marshmallow filling:

1. In a large mixing bowl, add the marshmallow and butter. Use an electric mixer and beat on high until fluffy and combined, about 5 minutes.
2. Add in the confectioners’ sugar, vanilla, and salt. Beat again until well integrated and fluffy, about 2 minutes.
3. Add the filling to a piping bag with a large round tip. Pipe the filling onto half of the cookies. Top each filled cookie with another cookie.