In a stand mixer, cream together the sugar, salt, butter, vanilla and egg.
In a separate bowl, sift together the flour, baking soda and cocoa. Add this to the first mixture alternately with the water and buttermilk.
Drop the batter by teaspoonfuls onto a greased non-stick cookie sheet. For easiest removal, line the cookie sheet with parchment paper. Bake the cookies at 400 degrees Fahrenheit for 7-8 minutes. Watch them closely. If your cookie cakes are smaller than ours, they will bake faster. Set the cakes aside to cool on the pan until you can move them easily to a wire rack to cool completely.
For the Oreo filling:
In a stand mixer, whip the softened butter and shortening on medium speed for 2-3 minutes. Add the vanilla and powdered sugar. Mix on low speed until the powdered sugar is all incorporated.
Add in the heavy cream, one tablespoon at a time, and beat the frosting on medium speed until the frosting has reached the consistency that you like. (I used 6-7 tablespoons of cream).
Add the Oreo cookies to the frosting, 3 at a time, and beat on medium-low speed until they are all broken up and blended in.
Choose two whoopie pies that are about the same size. Spread the Oreo frosting between the two cakes. This should make about 10-15 whoopie pies, depending on the size you make the cakes.
Store at room temperature in an airtight container.
*This frosting recipe makes enough for a nice amount of filling on each pie. The pies will be thick! If you don't want as much filling, you may be able to halve the frosting recipe.The calories shown are based on the recipe making 15 pies, with 1 serving being 1 pie. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**