Preheat oven to 180 degrees celsius (fan-forced). Line an 18X28cm rectangular slice tin with baking paper and set aside.
Melt the butter in the Thermomix bowl on Speed 2, 100 degrees, 3 minutes.
Add the plain flour, brown sugar and coconut and mix on Reverse, Speed 4, 15 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.
Place the white chocolate melts into a clean Thermomix bowl. Grate on Speed 8 for 10 seconds. Scrape down the sides of the bowl, add the vegetable oil and melt on 60 degrees, Speed 2, 3-4 minutes (or until completely melted).
Pour the chocolate topping over the cooled caramel layer.
Cut into slices just before the chocolate topping sets completely.Store in an airtight container.