Preheat oven to 160°C (Gas mark 2/325°F). Line 2 baking trays with baking paper.
Place the butter and sugar in a large mixing bowl, then whisk together until pale and fluffy.
Add the vanilla extract and whisk again until well combined.
Sieve in the flour. Whisk again until the mixture comes together in a dough.
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. Our 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 biscuits to cool on the baking trays for 10 minutes before transferring to a cooling rack to finish cooling.
Once the biscuits 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.
Pipe a zig-zag pattern onto each cookie to make a Christmas tree.
Working quickly, top with mini rainbow chocolate beans to decorate. Leave the icing to dry and set.
Once the icing has completely dried, store the biscuits in an airtight container, layered with baking paper to separate them, until ready to serve.