Chocolatey whoopie pies with marshmallow filling

Whoopie Pies are a trend that has gone completely past me. While I'm still swimming on the cupcake wave, first came the cake pops and finally the whoopie pies. Both are actually rather suspicious to me. Are Whoopie Pies maybe out again? As always, I jump on the train relatively late. As they are apparently still on everyone's lips, I had to go now and bake Whoopie Pies myself. Otherwise I can not have a say anyway. They are supposed to be very similar to cupcakes, only in biscuit look and with the frosting in the middle.
Well, I'm always open to new things (even though it's not that new anymore) and try it So, to whoopie pies.
I'm looking for classic recipes and find a kind of standard recipe in various variations of dark chocolate sauce with a fluffy marshmallow filling.
The recipe reminds me very much of my (or Marthas ...) chocolate cupcakes with marshmallow frosting and filling. So I change it to my own taste and go.
Perfect, so let's start the direct comparison between Whoopie Pies and Cupcakes.

Chocolate Whoopie Pies with marshmallow filling

for 8 Whoopie Pies
For Whoopie Pies:
1 room-warm egg (size L)
150 g Demerara or other light raw cane sugar
a few drops of vanilla extract
150 g buttermilk (or 150 ml milk mixed with 1 tbsp lemon juice, leave for 5 minutes)
75 g melted butter
200 g flour
75 g unsweetened, untreated cocoa powder (eg from Valrhona)
3/4 tbsp soda
For the filling:
150 g soft butter 250 g icing sugar, finely sieved
213 g (1 glass) marshmallow cream (spread)
Egg, sugar and vanilla extract with the hand mixer or in the food processor for a few minutes Whip up the cream until the sugar dissolves well.
Add the buttermilk and mix carefully.
Add the butter and mix thoroughly again.