- 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.