Line your 9-inch (20cm) square tin with non-stick baking paper, make sure you have excess to make removing it from the tin easier. Preheat the oven to 160c Fan (180c).
Cream together the softened butter and castor sugar until light and fluffy.
Once fully combined, add the gluten-free plain flour and xanthan gum( if you are using it). Bring the mixture together it should look like crumbly sand. You can use your hands if you wish.
Next press the dough into your tin, make sure you press it right into the corners. Prick it all over with a fork then bake in the oven for 20-22 minutes until golden. Leave to cool.
Now make your caramel. Put all the ingredients into the saucepan and melt together stirring continuously, making sure nothing sticks to the bottom of the pan. Turn up the heat and the mixture will start to bubble. Keep stirring it for around 5 minutes. The caramel should go a bit thicker and a darker brown colour.
Pour the caramel over the biscuit base and leave to set for 2 hours.
Once cooled, melt the chocolate in separate bowls. Use a bain-Marie (put a bowl on top of a saucepan full of boiling water and add chocolate). Melt the milk chocolate first and pour onto the shortbread, next add the white making a swirly pattern and leave to set.
Once chocolate is set remove from the tin and slice into squares.