Preheat oven to 180 degrees celsius (fan-forced). Grease and line a 23cm square baking tin with baking paper and set aside (leave a 5cm overhang on the sides).
Place the plain flour, icing sugar, brown sugar, salt and butter into the Thermomix bowl and mix for 10 seconds, Speed 8. Mix on Interval/Knead function until the mixture turns into crumbly little balls.
Pour the mixture into the prepared pan and use a spoon to spread out evenly and press down firmly. Bake for 15 minutes or until lightly golden. Allow to cool in the pan.
To make the caramel, place the sweetened condensed milk, brown sugar, golden syrup, vanilla extract and butter into a clean and dry Thermomix bowl. Mix on 80 degrees, Speed 4, 8 minutes. Check the consistency and if it's not considerably thicker, continue cooking (same temperature and speed) in 1 minute intervals until thick.
Pour the caramel over the shortbread base and place into the fridge to set.
Remove from the baking pan and place onto a chopping board. Cut one long line down the middle and then cut each half into approximately 15 small fingers. Place back into the fridge for 30 minutes.
Place the milk chocolate into a clean and dry Thermomix bowl. Melt for 4 minutes, 60 degrees, Speed 2 (or until melted). Dip the caramel shortbread fingers into the melted chocolate and allow any excess to drip off.
Place onto a flat baking tray and place back into the fridge to set. Optional: drizzle with extra melted dark chocolate.
Store in an airtight container in the fridge for up to 5 days.