Pre-heat your oven to 180 degrees, 160 degrees for a fan oven.
Start by combining the base ingredients together in a large mixing bowl.
Using a fork, mash the margarine into the dry ingredients until you have something that resembles breadcrumbs.
Push the mixture into a greased 20cm x 20cm square tin and place in the pre-heated oven for 30 - 40 minutes until firm and starting to slightly brown.
Leave to cool in the tin and meanwhile make the caramel.
To make the caramel, place the coconut milk, granulated sugar, brown sugar, margarine and vanilla in a large pan over a low heat and stir until the sugar dissolves.
Once dissolved, turn the heat up to medium and, stirring continually with a whisk, allow to simmer for about 20 minutes or until thickened and reduced in volume by about half.
Add the dark chocolate and stir until combined.
Pour the thickened mixture over the cooled shortbread and leave to set, ideally in the fridge once cooled a little.
If you can wait, it is best to leave it overnight before cutting so that the caramel sets properly, then slice into pieces the size you require (big for adults, tiny for kids ;)).