Preheat the oven to 180c / 360F.
Place the room temperature softened butter into a large bowl. Use an electric whisk, and whisk the butter for around 2 minutes untile it is a very pale yellow and light and fluffy.
Add the powdered sugar to the butter. Starting on a low speed, whisk the sugar into the butter. Whisk for around two minutes until incorporated.
Add the egg yolk and vanilla extract to the butter and sugar and whisk to combine.
Scrape off the excess mixture from the beaters into the bowl, but leave some on. Set the whisk to one side as you will need a small amount of the butter mix for later in the recipe.
In another bowl, mix the flour and salt. Add this dry mix to the butter and sugar mixture in the large bowl.
Using a spatula, gently start folding the mixture together. Don't stir or use a whisk as this can cause the gluten to activate. Fold well until you have formed a dough. This will take several minutes and the finished dough will be sticky.
Measure the dough and split it in half i.e. if it weighs 360g, make two dough balls of 180g each and place in two separate bowls.
Sieve the cocoa powder into a small bowl. Scrape the butter mix that was left on the beaters from earlier into the cocoa. Stir til incorporated.
Place the cocoa mixture into one of the bowls of dough and fold in gently til well combined. Again, do not over mix. These will be your chocolate cookies.
Place a 4B piping tip (open star) into a silicone piping bag and fill with the vanilla cookie dough. Pipe the dough on to half of one cookie sheet, and then half on another. Clean the piping bag (or use a new one!) and repeat with the chocolate dough.
Place a baking sheet with the cookie dough in the preheated oven. You will need to bake them in a couple of batches - do one tray, then the next.
Bake for 4 to 6 minutes until the vanilla cookies are a very light brown and the cookies are firm. Timings will vary between ovens and the baking sheets that you use, so keep an eye on them!
Remove the cookies from the oven and let cool fully before decorating.