Pre-heat the oven to 325°F (170°C) on convection setting (fan assist*). Line 3 baking trays with parchment and set aside.

In a food processor thoroughly mix the flour, sugar, cocoa powder, baking soda and baking powder and salt. Add the butter and pulse until it resembles fine breadcrumbs.

Add the egg and continue processing until dough comes together in a ball. This take seconds.

Scoop 1 level tablespoon of dough and place on the baking sheet approximately two inches apart. Flatten down the balls slightly with your hand. The dough will yield about 40 cookies.

Bake for 8 - 10 minutes. Immediately remove from the oven and bang on the counter top to create crinkles on the cookies. Allow to cool on the trays while you make the frosting.

Using a stand mixer (or electric hand mixer) whisk the butter on high speed for 2-3 minutes. Lower the speed, gradually beat in the sugar a spoonful at a time, followed by the vanilla. Turn the mixer on high and beat for 3-4 minutes until filling is light and fluffy.

In a pastry bag with a ½ inch, round tip, pipe 2-3 teaspoons of buttercream into the center of one cookie. Place another cookie, equal in size to the first, on top of the cream. Lightly press, to work the filling evenly to the outsides of the cookie. Continue this process until all the cookies have been sandwiched with cream.

Dunk generously in a large glass of milk. Store cookies in an airtight container at room temperature for up to 2 days. You can keep assembled homemade OREO cookies refrigerated if the buttercream gets too soft.