Classic Whoopie Pies

Preheat oven to 350 degrees F. In a large bowl, beat shortening with an electric mixer on medium to high speed for 30 seconds. Add sugar, baking soda, and salt. Beat until combined, scraping bowl occasionally. Beat in buttermilk or sour milk, egg, and vanilla until combined. In a medium bowl, stir together the flour and cocoa powder. Beat in as much of the flour mixture as you can with the mixer. Using a wooden spoon, stir in any remaining flour mixture.