Grease an 18 cm (7 inch) round cake tin or similar. Put the plain chocolate and milk in a bowl set over a pan of simmering water, making sure the bowl does not touch the water, and leave until the chocolate has melted, stirring occasionally. Stir in the butter. Remove the bowl from the heat and leave until cool, but not solid.
Mix the biscuit pieces with the white and milk chocolate buttons, then stir the mixture into the melted chocolate, pour it into the tin and squash it down gently. Put in the refrigerator for at least 3 hours until firm, then cut into wedges.