Melt the butter with the golden syrup and cocoa powder in a bowl over a pan of simmering water. Stir in the biscuits, marshmallows and Maltesers.
Grease a 25 x 18 cm (10 x 7 inch) tin. Chill the mixture in the prepared tin for 15 minutes.
Melt the milk and plain chocolate together in a bowl over a pan of simmering water (see
Chill for 20 minutes, then cut into slices or squares.