In the bowl of a stand-up mixer (with the paddle attachment) or a large mixing bowl (using an electric hand mixer), add the butter and sugar. Beat them together until light and fluffy, about 1 minute. Scrape down the sides and mix again for a few seconds. Add the egg and vanilla extract and beat once more until combined.
Sift in the dry ingredients: all-purpose flour, cocoa powder and salt. This step is unfortunately necessary because cocoa powder is often times lumpy.
Cover the sides of the bowl with a towel so the flour won’t escape. Mix on low speed for 30 seconds. Remove the tea towels and continue to mix for another 30 seconds. The dough will be ready when all the flecks of flour disappear and it begins to clump around the mixer.
Divide the dough between two sheets of plastic wrap. Press the dough down and wrap it in plastic wrap. Transfer to the fridge to chill for 1 hour or up to 1 day.
Preheat the oven to 350 degrees. Remove the first sheet of dough from the fridge. If the dough is too stiff/cold to roll out, let it sit for 10 to 15 minutes, until it’s rollable. Flour your work surface and rolling pin. Roll the dough out to a 1/4-inch thickness. Using your cookie cutter, stamp out cookies. Transfer the cookies to a parchment-lined baking sheet and place in the freezer to chill for 15 minutes (this helps the cookies keep their shape when baked). Repeat the rolling out and stamping out process with the second sheet of dough. Repeat chilling process too.
Bake the cookies, one baking sheet at a time, for 8 to 10 minutes, until the center of the cookie no longer looks wet. The baking time will vary depending on the size of your cookie.
Allow the cookies to cool completely before decorating