Preheat the oven to gas mark 4 or 180 degrees Celsius.
Cream together the butter and caster sugar in a large mixing bowl using a wooden spoon or electric mixer.
Add the flour until it forms a biscuit dough. Press into the base of a lined 20 x 20 cm square tin.
Prick it with a fork, then bake for 15-20 minutes until golden.
While the shortbread cools, place the butter, sugar, syrup and condensed milk into a pan and heat slowly on the hob.
Once it begin to bubble, continue stirring for a further 5 minutes until it thickens. Pour onto the top of the shortbread and leave to set for half an hour or so.
Melt the milk chocoalte, and pour onto the caramel. Melt the white chocolate and pour this into parallel lines across the milk chocolate. Drag a cake tester or skewer across the white chocolate lines to create a feathered effect. Leave to set, slice into squares and serve.