Over a pan of gently simmering water, melt together the milk and dark chocolate, butter, golden syrup and half of the marshmallows in a heatproof bowl.
Remove from the heat and stir in nearly all the popcorn and most of the remaining marshmallows, reserving some of each for the top.
Pour the mixture into a lined 20cm square cake tin and scatter the reserved marshmallows and toffee popcorn over the top.
Melt the white chocolate in a heatproof bowl over a pan of gently simmering water. Use a spoon to drizzle the melted chocolate over the top.
Set in the fridge for 3 hours before cutting into 25 squares to serve.