Do this for each ball, then serve within half an hour, otherwise the cocoa will moisten and turn a very dark brown, far less appetizing.
Remarks
You can replace the sponge cookies with boudoirs.
This is a recipe that can easily be made into filled balls: As you roll them, insert a small piece of fruit purée or jam (for example) in the middle, which you've hardened in the freezer in small heaps.