Preheat oven to 375 degrees F (190 degrees C). Grease cookie sheets.
Sift together the flour, cocoa, baking powder, baking soda, and salt; set aside.
In a large bowl, cream together the butter, sugar, and brown sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the buttermilk.
Drop batter by rounded spoonfuls onto prepared cookie sheets. Bake for 8 to 10 minutes in the preheated oven. Allow cookies to cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.
To make the filling, cream together the butter and shortening. Gradually beat in the powdered sugar, then stir in the vanilla and enough milk to achieve a spreading consistency. Spread between cooled cookies.
Interesting Facts
Whoopie pies are believed to have originated in Pennsylvania in the 1920s.
The first published recipe for whoopie pies appeared in a church cookbook in 1925.
The traditional recipe uses shortening in the filling, but you can substitute butter if desired.