Add the condensed milk and butter to a saucepan and heat gently on the stovetop until the butter has melted.
In a bowl, mix the crushed biscuit crumbs together with the cocoa powder.
Add the condensed milk to the biscuit crumbs and stir together until well combined.
Put the mixture in the fridge for about 1 hour to cool down completely.
Roll the mixture into balls (about the size of a walnut) and place on a baking tray which has been lined with baking paper. Return to the fridge to chill.
Gently melt your chocolate in a heatproof bowl that has been placed over a pan of simmering water.
Using a kebab stick or something similar to hold your truffles, dip them in the melted chocolate and place back on the baking tray. Return to the fridge to chill until the chocolate is firm.