Preheat oven to 350 degrees F (175 degrees C).

Whisk flour, cocoa powder, salt, baking powder, and baking soda in a bowl. Beat 1 cup butter and white sugar in a separate large bowl, with an electric mixer on medium speed, until mixture is creamy. Beat eggs and 2 teaspoons vanilla extract into butter mixture.

Stir half the flour mixture into wet ingredients, followed by milk; stir remaining flour mixture into dough. Drop dough by tablespoonfuls onto baking sheets. For 25 whoopie pies, make 50 cookies.

Bake cookies in the preheated oven until set and lightly browned, about 10 minutes. Let cool completely.

Beat 3/4 cup butter with confectioners' sugar in a bowl, using an electric mixer; beat 2 teaspoons vanilla extract and marshmallow creme into filling. Spread at least 1 teaspoon filling on the bottom of a cookie and top with a second cookie. Repeat with remaining cookies and filling.