Preheat oven to 375°F.
Sift together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
In another bowl, beat the softened butter, sugar, eggs, vanilla, and milk until combined.
Add the dry ingredients and mix until just combined.
Let the dough rest for 9 minutes.
Drop rounded tablespoons of the dough onto a baking sheet.
Bake for 8 minutes. Let cool completely.
For the filling, beat the butter, powdered sugar, marshmallow fluff, and vanilla until fluffy.
Fill the cooled whoopie pies using the same size spoon.
Makes approximately 24 Whoopie cakes, ready to eat.
