Melt the milk chocolate in a Bain Marie (double boiler).
Line a square or rectangular tin (mine is 8"x 8") with baking paper or foil. Pour the melted chocolate in and gently tilt so that the chocolate evenly coats the whole tin.
Break up the shortbread into fairly big pieces and sprinkle evenly over, pushing down a little so that they are half submerged in the chocolate. Place in the fridge to set.
Dollop the caramel over the hardened chocolate and shortbread and using a knife, gently spread it all over.
Heat the white chocolate then pour over the top, using a knife to gently spread it all over, making sure you evenly coat the other ingredients. There should be enough chocolate to complete cover everything.
Allow to set completely. PLEASE NOTE: do not put it in the fridge as the chocolate will splinter when you cut it. This is great for a normal bark, but not for this recipe.
When ready, take a sharp knife that has been held under a hot tap. Place the knife where you want to cut the bark and let it slide through rather than cutting it. You will have a little resistance when you reach the shortbread. Some of the white chocolate will crack but this often unpreventable. I cut mine into 9 squares (but they were big) so 12 would work just as well. Enjoy!