Cream together the softened butter, sugar, and brown sugar until light and fluffy (about 2-3 minutes).

Next add the sourdough discard, egg yolk, and vanilla until smooth.

To the same bowl add the flour, black cocoa powder, baking soda, and salt.

Mix until a soft dough is formed.

Cut 1 large piece of parchment paper and dust it with flour. Roll the dough out on the parchment paper as thin and even as possible—about ⅛ inch thick. Tip: Lightly flour your rolling pin to prevent sticking.

Place the rolled out dough on a large baking tray, cover it and place it into the refrigerator for 1-24 hours (The longer chill time deepens the flavor and makes the dough much easier to handle).

Once ready to bake the sourdough Oreo cookies, preheat the oven to 350 degrees F.

Remove the sheet of dough from the fridge and place it face-down on a lightly floured surface and gently peel off the parchment paper. (You can reuse that same parchment to line your baking sheet.)

Cut as many 2-inch circles as possible and transfer them to your baking sheet.

Any additional dough will need to be rolled out and cut. If the dough seems sticky add some flour or place it in the freezer to firm up.

Bake the sourdough Oreo cookies for 10-12 minutes for that crispy finish (if you'd prefer a softer cookie bake for 8-10 minutes).

Allow them to cool completely on the pan before filling. They crisp up as they cool.

Beat together the shortening and softened butter until fluffy. Slowly beat in the sifted powdered sugar, vanilla, and a teaspoon or 2 of milk/heavy cream for desired consistency.

Give the filling a taste test, if it is too sweet add a pinch of salt.

Pipe or spoon ~1 tablespoon filling onto one cookie (bottom side up) and gently press a second cook on top.

Once all sandwich cookies have been assembled, place them in the fridge for 10 minutes to set and enjoy!