Generously grease a 28x18cm tray with butter and set aside.
In a pan, melt the butter, sugar, cocoa powder.
Meanwhile, crush the disgestive biscuits, either in a sandwich bag with a rolling pin, or blitz in a blender. Add to the melted mixture and stir until combined.
Transfer the base mixture to your prepared tray and using a wooden spoon, press the mixture into the tray, ensuring you get right into the corners and have an even finish.
Place the base into the fridge to chill for 1 hour.
As the base is about to finish setting, start to prepare the filling by sifting the icing sugar into a large bowl. Add 1 tsp of hot water at a time until you have a spreadable consistency; 3-4 tsp of water should be enough but add more or less as required.
Add the Valencian orange extract and orange food colouring (optional) to the icing and mix well until combined. Spread the icing evenly over the biscuit base and return to the fridge for another 1 hour to set.
Once cooled, melt the dark chocolate in a microwave (or in a pan) and add Valencian orange extract. Then evenly spread over the filling.
Now melt the white chocolate and then pour in lines down the cake. With a cake tester or skewer, drag through the white chocolate in opposite directions across the cake to give a feathered effect.
Allow bake to set in fridge for min 1 hour before cutting into 12 equal slices. When cutting, allow the bake to come to room temperature and use a knife soaked in boiling water to avoid the chocolate from cracking.