Preheat the oven to 180°C and line a 30cm x 20cm (12in. x 8in.) baking tin.
Mix the flour and caster sugar in a large bowl, then add the butter and rub the mixture together with your fingers until it resembles breadcrumbs.
Bring the mixture together so it forms a dough then press it evenly into the base of the prepared tin.
Prick the dough a few times with a fork then bake for 20 minutes, until lightly browned, then leave to cool in the tin while you make the caramel.
Put the butter, light brown sugar and two cans of 397g condensed milk into a medium pan and gently heat until the sugar has dissolved.
Bring the mixture to the boil, then reduce the heat and simmer, stirring at all times, for about 5–10 minutes until the mixture has thickened.
Immediately pour over the shortbread base and leave to cool for an hour.
Put the milk and dark chocolate into a heatproof bowl and set it over a pan of simmering water. Let them melt, stirring the two chocolates together, then pour them over the cooled caramel.
Repeat the above in a clean bowl and melt the white chocolate but, once melted, add a few drops of pink food colouring and mix it through.
Drizzle the now pink chocolate over the top of the milk/dark chocolate then,
using a toothpick or the tip of a knife, swirl the two chocolates together until you have a marble effect.
Leave aside to cool, then cut into 12 squares and serve.