Preheat your oven to 180 degrees and line two trays with baking paper.
Place the butter, caster sugar and vanilla extract into your Thermomix bowl and mix for 30 seconds, speed 3.
Scrape down the sides of the bowl and mix for a further 10 seconds, speed 3 or until white and fluffy.
Add the flour, egg and extra egg yolk and mix for 10 seconds, speed 4.
Select the kneading function and mix for 2 minutes.
Turn the dough out onto your bench and bring together to a ball. Wrap in glad wrap and pop in the fridge for 30 minutes - or if freezing, wrap in glad wrap and pop into the freezer.
Roll out dough (thickness of 5mm) between two sheets of baking paper and using your choice of a cookie cutter (I use a 7cm round cutter for this recipe), cut out shapes and put onto baking trays lined with baking paper (add hundreds and thousands at this point if you wish).
Bake in oven for 10-12 minutes or until golden at the edges. Allow the biscuits to cool on the tray for 10 minutes before transferring to a wire rack to cool completely.