Preheat your oven to 160 degrees and line two trays with baking paper.
Place the butterfly attachment into your Thermomix bowl and add the softened butter and caster sugar. Mix for 1 minute, speed 3 to combine.
Scrape down the sides of the Thermomix bowl and mix for a further 30 seconds, speed 3 or until the mixture is pale and creamy.
Remove the butterfly attachment and add the plain flour and mix for 2 minutes on the dough function to bring the mixture together.
Turn the mixture onto a lightly floured bench and gently bring it together to form a ball with your hands.
Cut the shortbread dough in half and wrap one piece in plastic wrap and pop it into the fridge until needed.
Roll the dough until it’s approximately 5mm thick and use an 8cm star shape cutter to shape the biscuits.
Transfer the shortbread to the prepared baking trays and bring together the remaining pieces of the dough, before repeating step 5 until you have used all of the shortbread dough - including the extra you have put aside in the fridge.
Bake the shortbread for 10 - 12 minutes or until the edges are just beginning to turn golden. Carefully remove the shortbread from the oven and allow it to cool on the baking trays for 15 minutes before transferring to a wire rack.