Melt 370g chocolate in a bowl set over a pan of simmering water. Line a large baking sheet with greaseproof paper and pour the chocolate over.
Soften the peanut butter in a microwave, then dollop it over the unset chocolate. Using a toothpick, spread the peanut butter around so it creates a pattern. Sprinkle over the popcorn. Melt the remaining chocolate and, using a small spoon, drizzle this over the entire surface. Place in the fridge to firm up for at least 1 hour.