I think this may have been an old Sainsburys recipe. I have simply swapped the milk chocolate for dark chocolate and reduced the amount of white chocolate. It is dead simple.
200g dark chocolate
100g white chocolate
60g toffee popcorn (or ordinary popcorn which reduces the sugar hit)
Line a 20cm x 30 cm baking tray with parchment
Melt the two types of chocolate separately over a bain maries and allow them to cool a little.
Pour most of the two types of chocolate into the tray, roughly swirling together.
Sprinkle over the toffee popcorn and then drizzle over the remaining dark and white chocolate.
Chill until firm.
Break into big chunks and scoff.