Preheat oven to 375 Degrees
Coat 2-3 baking sheets lightly w/ non-stick cooking spray and line w/ parchment paper. Set aside
In a medium bowl, whisk together flour, cocoa, baking soda, baking powder, and salt. Set aside
In a large bowl using a handheld mixer, (or your kitchenaid) beat softened butter and sugar together until light and fluffy. (about 3 minutes)
Beat in egg and vanilla until mixed
Slowly beat in milk until mixed
Use a rubber spatula and fold in the dry ingredients into the wet ingredients until just combined
Using a small scoop, drop 1-2 tbsp worth of batter onto the prepared baking sheet. Leave 2in between each scoop of batter.
Bake in preheated oven for 8-10 min, or until the tops begin to firm up. Remove from oven and allow to cool on baking sheet for 1 min, then transfer to a wire cooling rack to cool completely.
While the whoopie pies are cooling, prepare the filling by beating together butter, confectioner's sugar, marshmallow cream, and vanilla until fully combined.
Using a piping bag or spoon, spread 2 tbsp worth of marshmallow buttercream onto the inside of one cake layer then gently press the other cake layer over top to make a sandwich.
