Add the condensed milk, cocoa and butter to a medium heavy based saucepan, bring to the boil, whisking constantly. Reduce heat to medium-low; cook 10 to 15 minutes, stirring constantly, until mixture is thick and shiny and starts to pull away from the bottom and side of the saucepan when pan is tilted. Pour mixture into a wide, shallow dish (this will allow it to cool faster) and place in the fridge for at least 2 hours or overnight.
Add the icing sugar to a small wide dish and add the the cocoa powder and espresso together in another small flat dish. Once the truffle mixture has chilled, use a melon baller to scoop out the truffles to give you 20 in total. Shape in your hands and as you go toss half through the icing sugar to coat and the other half into the cocoa espresso coating.
Store in a container with a tightly fitted lid, at room temperature up to 2 days or in the refrigerator up to 1 week.