First make the orange jelly to give it time to set. You might want to do this the day before. Make the jelly up as directed with boiling and cold water in a jug. To give a better set, I added additional gelatine. Soften 1 leaf of leaf gelatine in cold water, squeeze it out, then melt the leaf by heating on a low heat in small bursts in the microwave. Stir vigorously and add to the jelly in the jug. Mix well then pour the jelly into a shallow baking tray lined with clingfilm to set in the fridge. Leave for a few hours until set.
To make the shortbread layer, leave the butter out until softened or warm up gently in the microwave. Then combine the caster sugar into the butter until soft and fluffy.
Add the flour and mix well. Press into a ball with your hands.
Line a square pan (I used a silicone one, approx 9 inch square) with baking paper. Press the mixture into the pan with the back of your hand. Bake for 25-30 minutes until golden brown. Leave it to cool completely.
When cool, cut your jelly to size and place it on top of the shortbread layer. This is quite tricky to do and mine broke in places. As the jelly layer will be covered in chocolate it doesn't really matter, just try to move it gently.
Break the plain chocolate into pieces and microwave in 30 second bursts on high, stirring in between, until fully melted. Allow it to cool a little then pour it on top of the jelly, spread it with the back of a spoon then put it back in the fridge to set.
Use a hot knife to slice into pieces and serve.