Preheat oven to 350°F.
For the batter: In a bowl, whisk together the flour, cocoa powder, baking powder, and salt.
In a separate bowl, cream together the butter, sugar, and salt until light and fluffy. Beat in the eggs one at a time.
Using milk, gradually integrate the wet and dry ingredients, alternating between the two, until a smooth dough forms.
Using a piping bag or use two spoons to form 20 equal-sized mounds on a baking sheet lined with parchment paper.
Bake for 10-15 minutes, then let cool completely.
Meanwhile, for the filling, beat together the marshmallow spread and butter. Stir in the powdered sugar, vanilla sugar, and salt.
Spread some of the filling on 10 of the cooled cookies and top with the remaining cookies.
