Preheat the oven to 170°C/325°F/Gas mark 3
Line your baking tin with baking paper on the bottom and sides of the tin.
In a bowl beat together butter and sugar until light and fluffy.
In a large bowl mix flour, cornflour, xanthan gum together until fully combined.
Add butter and sugar mixture to the large bowl with the flour mixture.
Add water and with your hands knead together and will resemble large crumbs but continue to knead as it will eventually form a shortbread dough.
Place your dough onto a floured surface. Using a rolling pin roll out your dough evenly until around the same size as your prepared baking tin. Then place in the baking tin pressing down firmly and with a fork prick the surface of the shortbread dough.
Place in the oven and bake for 20-25 minutes until slightly golden on the edges but still pale on top.
Once baked leave in the baking tray to slightly cool the shortbread may still be soft to touch when removed from the oven but will firm up as it cools. Start the caramel layer when the shortbread has cooled and firm.