Preheat your oven to 160°C (Gas mark 2/325°F). Line 2 baking trays with baking paper.
Pop the butter and sugar into a large mixing bowl, then whisk together with an electric whisk until pale and fluffy.
Add the vanilla extract and green gel food colouring and whisk again until well combined.
Sieve in the flour. Whisk again until the mixture comes together to form a dough (You may need to pull the dough together with your hands as it's quite crumbly).
Roll the dough out on a lightly floured surface to around 1cm thick. Cut a triangle from thick card and use as a template to cut the dough into triangles. My triangles were approx. 10cm x 6cm in size.
Carefully transfer to the baking trays, then bake for around 15 minutes until golden.
After removing from the oven, leave the cookies to cool on the baking trays for at least 10 minutes before transferring to a cooling rack to finish cooling. Be careful, these cookies are very soft and crumbly whilst still warm.
Whilst the cookies are cooling, pop the chocolate stars onto a piece of baking paper and spray with the edible gold or silver lustre.
Once the cookies are completely cool, make the icing:
Sieve the icing sugar into a bowl then add the food colouring with a splash of water. Stir well until the sugar has dissolved and the icing is completely green. Add a little more water if needed to get the correct consistency of icing – it should be a thick paste suitable for piping.
Spoon the icing into a piping bag with a thin round piping tip (if you don't have a piping tip, just use a disposable piping bag or even a freezer bag, and snip off the tip for piping).
Pipe a zig zag pattern onto each cookie to make a Christmas tree.
Working quickly, top with sprinkles of your choice to decorate. I used mini rainbow chocolate beans, hundreds and thousands and silver and white sugar pearls.
Finish each cookie with a chocolate star, then leave the icing to dry and set.
Once the icing has completely dried, you can store the cookies in an airtight container, layered with baking paper to separate them, until ready to serve.