Preheat the oven to 350 degrees. Line two baking sheets with parchment paper.
In an electric mixer, beat the butter and sugar together until just incorporated. Do not over-mix at this stage or the cookies may spread while baking. Add the egg and vanilla. Mix on low speed, stopping to scrape down the sides of the bowl intermittently as needed.
In a medium bowl whisk together the flour, cocoa powder and salt. Add to the butter and egg mixture. Mix on low speed until a dough is formed and there are no longer any streaks of butter in the mixing bowl. The dough will clump around the paddle attachment but that's ok, it's normal and a good sign your dough is the right consistency. With your hands, form the dough into a ball.
On a clean counter-top, roll out your dough to ¼-inch thickness. Cut out desired shapes from the cookie dough and carefully transfer to your baking sheets. Bake the cookies for 15 minutes, rotating pans and switching racks halfway through to endure even baking. Let them cool on the baking sheet for 5 minutes and then transfer to a cooling rack to cool completely. Frost with frosting and decorate with sanding sugar, sprinkles, etc.
For the Buttercream Frosting
Beat butter, sugar and milk about 5 minutes. Beat in extracts.
Notes
Makes 16, 4-inch Sandwich Cookies.
Tip: These cookies are best eaten within 24 hours or the same day they are made. They are still very good but not be quite as crisp the next day.
Recipe by DessertedPlanet.com at https://dessertedplanet.com/dark-chocolate-sugar-cookies-aka-homemade-oreos/