Preheat oven to 180 degrees celsius (fan-forced). Line a 20X28cm rectangular slice tin with baking paper and set aside.
Melt the butter in the TM bowl on Speed 2, 100 degrees, 3 minutes.
Add the plain flour, brown sugar and coconut and mix on Speed 4, 10 seconds. Scrape down the sides of the bowl and repeat for a further 15 seconds or until completely combined (use the spatula to help mix).
Press the mixture firmly into the base of the prepared tin and smooth with a spoon. Bake for 15 minutes or until lightly golden.
To make the filing, place the golden syrup, condensed milk and butter into a clean and dry Thermomix bowl. Mix on 80 degrees, Speed 4, 8 minutes.
Pour the caramel over the cooked base and return to the oven for a further 15-20 minutes or until golden. Remove from the oven and set aside until cool then place in the fridge to cool completely.
Place the chocolate melts into a clean TM bowl. Grate on Speed 8 for 10 seconds. Scrape down the sides of the bowl, add the vegetable oil (or coconut oil) and melt on Speed 2, 80 degrees, 4 minutes (or until completely melted).
Pour the chocolate topping over the cooled caramel layer (see notes).
Cut into slices just before the chocolate topping sets completely.
Store in an airtight container for up to 5 days.