Preheat oven to 160 degrees celsius (fan-forced). Line two large flat baking trays with baking paper and set aside until needed.
Place the icing sugar into the Thermomix bowl and sift for 10 seconds, Speed 8. Scrape down the sides of the bowl.
Weigh the butter and plain flour into the Thermomix bowl. Mix 10-15 seconds, Speed 6, and then knead for 1 minute, Kneading function.
Tip the dough out onto a lightly floured bench or Thermomat, and gently knead it into a ball, taking care not to overwork it. Roll out to ½ cm thick and use a round cookie cutter to cut circles out of the dough.
Place the biscuits onto the prepared trays and bake for 12-15 minutes or until only just very lightly golden (do not overcook).
Allow to cool on the trays for 10 minutes before transferring to a wire rack to cool completely.
Place the Caramel Top 'n' Fill into a small bowl and mix with a spoon until smooth and glossy.
In a clean, completely dry Thermomix bowl, melt the chocolate for 2 minutes, 50 degrees, Speed 2. Stir with a spatula and then melt for a further 2 minutes, 50 degrees, Speed 2 or until completely melted.
Spread a small amount of Caramel Top 'n' Fill over the top of the cooled biscuits and then top with a small amount of melted dark chocolate. Set aside until the chocolate layer has set.
Once the chocolate has set, the Twix Cookies are ready to serve! They can be stored in an airtight container at room temperature for up to 5 days (see notes).