Preheat your oven to 180°C (350°F), grease an 8-inch square baking tin, and line it with parchment paper. In a mixing bowl, cream together the softened butter, icing sugar, and vanilla extract for about 5 minutes until light and fluffy. In another bowl, whisk together the plain flour, corn flour, and salt. Gradually incorporate the dry ingredients into the butter mixture and stir until a dough is formed. Press this dough evenly into the prepared tin, using damp hands to smooth the surface. Bake for 35 to 40 minutes, until the top turns a light golden shade, then remove from the oven and allow to cool completely.

To prepare the caramel, combine the butter, condensed milk, sugar, and golden syrup in a large saucepan over low heat. Stir continuously until the butter has melted and the sugar is fully dissolved, with no visible granules remaining. Increase the heat slightly to medium, and while stirring constantly, cook until the mixture thickens and adopts a pale golden hue with a fudge-like consistency. Stir in the salt once it reaches the desired texture. Pour the caramel evenly over the cooled shortbread layer and let it sit for about 30 minutes to firm up.

Once the caramel is set, melt the dark and white chocolates separately using a microwave. Spread the melted dark chocolate over the caramel layer. Drizzle the white chocolate over the dark layer and use a skewer or knife to create a marbled effect. Allow the entire dessert to set either in the refrigerator for an hour or at room temperature overnight. Once firm, slice into squares and serve.