Melt together the butter and milk in a saucepan over a medium heat. Combine the sugar and cocoa, then add to the milk and cook for 5 minutes, or until the sugar has completely dissolved.
Remove from the heat and stir in the oats, pistachios, vanilla extract and half the coconut.
Sprinkle the remaining coconut over a plate and drop spoonfuls of the mixture into the coconut; roll into balls to coat. Transfer to a tray lined with baking parchment, then chill in the fridge for 15 minutes to set.