Line a baking sheet with parchment paper. This will help thee cookies not to stick to the pan.
In a medium sized bowl, combine graham cracker crumbs, flour, and baking powder, set aside
In a separate bowl, combine milk, vanilla, baking soda, and vinegar, set aside
In a stand mixer, combine 1/2 cup butter and brown sugar using the paddle attachment and cream together for 2 minutes.
After creaming the sugar and butter together for 2 minutes, add the eggs in one at a time.
Beat for 1 minute.
Slowly add in milk mixture and flour mixture, and mix until just combined.
Scoop mixture into a gallon sized ziptop bag, and cut the tip.
Pipe 2 inch circles of batter, it will spread, so make sure to give them space. (9 to a pan is the most I would do, you should get 24-26 total)
Bake 8-10 minutes, let sit on pan for 1-2 minutes, then move to a cooling rack.
Marshmallow Buttercream
While the whoopie pie shells cool, mix together all of the marshmallow buttercream ingredients in a large bowl, using a hand mixer, and pop in the fridge for a few minutes to allow to set up a bit.
Ganache
In a microwave safe bowl, heat 1/4 cup heavy whipping cream for 45 seconds to 1 minute
Pour the chocolate chips into the whipping cream, and stir until melted and shiny!
Add salt.
Assembly
Take a "cookie" and scoop 1 Tablespoon of chocolate ganache on top, and spread, then add 1 Tablespoon of the marshmallow buttercream, and top with another "cookie".
Store in an airtight container for up to a week!
Notes
Recipe adapted from Cookies and CupsTo keep, place on parchment paper and refrigerate. Bake on a metal baking sheet, and don't forget to let cool on sheet to get best results.