Preheat oven to 375-degrees.
Lightly grease 2 cookie sheets; set aside.
Sift together the flour, cocoa powder, baking soda and salt; set aside.
In a large mixing bowl cream the shortening and sugar together, until light and fluffy. Add the egg and vanilla and beat. Add dry ingredients, alternately with the buttermilk; starting and ending with the flour. Mix well.
Drop batter by tablespoonfuls (I made BIG cookies) onto greased cookie sheets. Try to make the mounds as round as possible, they'll look better when they're done. Whatever shape you drop the dough, that's the shape the cookie will be. The dough isn't real runny, but it's not real firm either. Space the cookies at LEAST 2 inches apart, they DO spread out quite a bit. Make sure you make an even amount of cookies--it takes 2 halves to make one Whoopie Pie.
Bake 8-10 minutes, or until cookies spring back when lightly touched and they're no longer shiny on top.
Leave the cookies on the cookie sheets for a few minutes, they'll fall apart if you remove them immediately. Cool completely before filling Cream the butter and shortening until light and fluffy. Beat in the marshmallow creme. Add vanilla and beat well. Add the confectioners' sugar and beat until smooth.
Spread 1 TBS of the filling onto one half of the cookies. Top with the other half. Of course, if you've made the cookies bigger, you'll need to put in more filling. I fill them as full as I want.
These freeze well, but they have to be air-tight or they get soggy. Carl likes 'em frozen! I've never tried it, but I'm sure these would make great ice cream sandwiches. Just put a slice of ice cream in between the cookies instead of the creme filling. The original recipes called for more shortening in the filling, it was waaaay too greasy. I like my filling better. You really need a little shortening to smooth the filling out--I've tried using totally butter or margarine, but it's not the same.