Place the butter into your Thermomix bowl and mix for 20 seconds, speed 2.
Scrape down the sides of your Thermomix bowl and add the caster sugar, baking powder and salt and mix for 20 seconds, speed 3. Scrape down the sides of the Thermomix bowl and mix for a further 20 seconds, speed 3 or until pale and creamy.
Add the egg, milk and vanilla extract and mix for 10 seconds, speed 3.
Scrape down the sides of the bowl and add the plain flour and mix for 10 seconds, speed 4.
Mix for a further 2 minutes using the dough/interval function.
Turn the dough out onto your bench and divide into two. Cover and place in the fridge for 45 minutes - 1 hour to chill.
Once the dough has chilled, preheat your oven to 170 degrees (fan forced) and line two trays with baking paper.
Roll the dough on your bench until it's approximately 4 mm thick and use the cutter of your choice to make shapes.
Place the cut out sugar cookies onto your prepared trays and continue until all of the dough has been used.
Bake for 8 - 10 minutes or until the sugar cookies are beginning to turn golden on the edge.
Allow to cool on the tray for 5 minutes before transferring to a wire tray to cool completely.
Once cool, decorate as desired.