In a medium saucepan, gently melt the milk chocolate, heavy cream, and butter, stirring until smooth. Finely grind the biscuits into crumbs using a food processor. Transfer to a bowl and stir in the melted chocolate mixture and cocoa powder until fully incorporated. Spread the mixture evenly between two sheets of parchment paper, pressing it into a thin rectangle about 10 by 12 inches/25 by 30 cm.

In a separate bowl, combine the coconut, powdered sugar, and butter until smooth and spreadable. Evenly spread this filling over the chocolate base. Using the parchment paper to guide you, roll tightly into a log. Keep the roll wrapped in parchment paper and transfer to the freezer for 1 hour, or until firm.

Remove from the freezer, unwrap, and coat the outside evenly with desiccated coconut. Slice into rounds with a sharp knife and serve cold.