If you have hot hands, using a pair of gloves may work best to keep the Nutella from melting as you stuff the cookies.
For maximum gooeyness, slightly underbake the cookies.
Cookies can be saved and stored in an airtight container for 3 days. To keep them fresher longer, add a piece of bread to the container and switch it out daily for maximum softness.
Cookies can be warmed in the microwave to melt the center again.