Preheat the oven to 180°C (350°F) and line a 9"x9" baking pan with baking paper.
Cream the butter and icing sugar together until it is light and fluffy.
Add the cornflour and flour and mix well.
Press the dough into the prepared pan and place in the oven for 20-25 minutes until the top is pale golden.
While the shortbread is baking, make the caramel filling.
Add the butter, condensed milk and golden syrup to a medium saucepan on a medium heat.
Allow everything to melt together before cranking the heat up to high. Allow the mixture to begin boiling, ensure you stir on and off to avoid the caramel sticking and burning at the bottom of the saucepan.
Boil for at least 5-7 minutes before removing from the heat.
Once the short bread is out of the oven, pour over the caramel and spread into an even layer before placing in the fridge to set.
Mine took 1.5-2 hours
Once set, melt the dark chocolate and white chocolate separately.
Pour the dark chocolate over the caramel and spread in an even layer.
Lightly drizzle over the white chocolate and using a knife, skewer or ends of a spoon, lightly swirl the white chocolate through.
You can either set the chocolate in the fridge for at room temperature. I prefer setting at room temperature as you can slice through the chocolate just before its completely hard to ensure you avoid cracks in the chocolate.
*Only add the milk if your dough is like breadcrumbs and only add enough milk for the dough to just combine.