Place flour, sugar, butter, vanilla, cocoa and salt into the Thermomix bowl and mix 6 sec/speed 8.
Add flour and knead 30 sec/Interval.
Tip out onto floured Silpat mat and gather into a smooth disc. Wrap and freeze for 30 minutes.
Pre-heat oven to fan forced 180°C.
Roll dough out as thinly as possible between two pieces of baking paper or silpat mats. Use plenty of flour to prevent sticking. Use MC or scone cutter to cut into rounds and transfer to lined baking sheets.
Bake for 8-10 minutes. Remove and cool before coating.
Place chocolate into clean, dry Thermo bowl and heat 15 min/37°C/speed 1. Scrape down sides of bowl if necessary. The chocolate should be very runny and liquid. Continue to heat if required. Add peppermint oil towards the end and allow it to incorporate as the chocolate is stirred. Pour the chocolate into a flat tray that you can place into the microwave if necessary to warm up the chocolate if it starts to set.
Using a fork, coat each cookie with chocolate, tapping the sides of the tray gently to remove excess chocolate. Place each dipped Thin onto a clean sheet of baking paper.
Leave Thins to set before transferring to a container. Keep refrigerated.