Pour the condensed milk into a saucepan. Add the cocoa powder and butter and stir over a low heat until the butter has melted.
Increase the heat slightly and stir as the mixture begins to boil. Continue to stir and boil the mixture for 10 minutes or until it has thickened and coats the back of the wooden spoon. Remove from the heat and cool for at least 15 minutes or until cool enough to handle.
Take small pieces of the chocolate mixture and roll into balls. Roll the sweets in either desiccated coconut, flaked almonds, grated chocolate or chocolate sugar strands, then chill until ready to serve.