1. Melt the peanut butter, butter, dark chocolate and syrup in a large saucepan over a low heat, stirring frequently, until completely smooth. Do not allow to simmer.
2. Add the marshmallows and popcorn and stir well, ensuring everything is evenly covered with the chocolate mixture.
3. Pour mixture into a greased 18cm square baking tin and smooth over with the back of a spoon.
4. Refrigerate for at least 2 hours and then slice into 16 squares - store in the fridge!