Heat the butter, runny honey and cocoa powder in a pan until melted. Add the crushed biscuits to the chocolate mixture and stir together until all the biscuits are coated with chocolate.
Pour into a greased 18 cm (7 inch) round tin. Chill for at least 30 minutes.
Melt the white chocolate in a bowl over a pan of simmering water (see
Chill for at least 30 minutes, then cut into 12 wedges.