Preheat your oven to 400 degrees and line 2 baking sheets with parchment paper.
With a handheld or stand mixer, cream together the sugar, salt, butter, vanilla, and egg. Scrape down the sides and bottom of the bowl.
In a separate bowl, whisk the flour, baking soda, and cocoa together.
Alternately add the dry ingredients to the first mixture with the water and buttermilk.
Drop a tablespoon of the batter at a time onto your prepared baking sheets about 2 inches apart.
Bake for 8 minutes.
Place cookies onto cooling rack to completely cool while you prepare the filling.
For the filling:
With a handheld or stand mixer, beat the softened butter until light and creamy. About 2 minutes.
Scrape down the sides of the bowl and add the powdered sugar and vanilla.
Turn your mixer on low. Once you're sure you won't be covered in powdered sugar, turn your mixer onto medium and add the heavy cream, one tablespoon at a time.
Beat for an additional 2 minutes.
Scrape down the sides of the bowl and turn mixer to low speed.
6. Add the Oreo cookies 1 at a time and beat until they've broken up into the buttercream.
Once your cookies are completely cool, fill the underside of one with the Oreo buttercream and sandwich together with another cookie.
If you don't have any buttermilk you can make your own! In a 1/2 cup measuring cup put 1 teaspoon white vinegar and then fill with milk. Let sit for about 5 minutes and use.
I put my filling into a piping bag fitted with a Wilton 4B tip.
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