In a pot add ur sugar and water close to boil, after boiled add the powder milk use mixer to mix it

Let it also boil for 3 min then Add the butter mix well, after mixed well add ur cocoa powder whisk it well with ur whisker, close for some min then open and gradually add the Corn flour after adding in some second it will just change to thick, keep mixing for about 5 min but you should reduce the heat after that pour in a container and store in the freezer, enjoy