First add your room temperature butter into you mixing bowl, along with your caster sugar. Cream these together with a wooden spoon or electric hand whisk.
To this add your egg yolk and mix this through, before adding your vanilla extract and mixing that all together.
You can then sift in your flour and add your cocoa powder. Mix this up until it's completely combined and resembles cookie dough. You can then add in your chocolate chips. I find it easier to mix these into the dough using my hands (just make sure they are clean).
To get all my cookies the same size and a perfect circle for my Christmas puddings I use a 3 inch cookie cutter. Place some parchment paper onto a baking tray and measure out 80g of mixture and push it into your cookie cutter.
Place your cookies into a preheated oven at 190 degrees Celsius (375 degrees Fahrenheit / gas mark 5) for around 12 - 15 minutes.
Place your cookies onto a cooking rack to cool before we decorate them.
To decorate your cookies mix up some Royal Icing using Royal Icing sugar. Place this into a piping bag with a number 4 piping nozzle. You can then pipe around the top of the cookie, making drip marks across the top half of the cookie.
Whilst this is starting to set, take some green fondant and cut out some holly, using a cutter and make some red balls for the berries. Attach these to the top of your cookie, before leaving them to set completely.