Preheat oven to 180 degrees celsius (fan-forced). Line an 18cm x 28cm rectangular slice tin with baking paper and set aside.
Melt the butter in the Thermomix bowl for 3 minutes, Speed 2, 100 degrees.
Add the plain flour, brown sugar and coconut and mix for 15 seconds, Reverse, Speed 4. 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 the back of a spoon. Bake for 15 minutes or until lightly golden.
To make the filling, place the golden syrup, condensed milk and butter into a clean and dry Thermomix bowl. Mix for 8 minutes, 80 degrees, Speed 4.
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 for 10 seconds on Speed 8. Scrape down the sides of the bowl, add the vegetable oil and melt for 3-4 minutes, 60 degrees, Speed 2, (or until completely melted).
Pour the chocolate topping over the cooled caramel layer.
Cut into slices just before the chocolate topping sets completely and store in an airtight container.