Just grab the jar, assuming its near-empty, and pour warm milk straight into it.
Screw the lid back on tight and shake.
When the Nutella is fully dissolved, pour your freshly-made Nutella cocoa into individual cups and serve.
If you want to go that route, maybe start with hot milk, and let it cool.
For a visual demonstration, check out the video above (skip to 00:33).
5 Minute Nutella Hacks| The Kitchy Kitchen (YouTube)