I thought I'd update this recipe since it's the most viewed on my blog. My previous recipe works best with Nutella produced & sold in Europe (at least, what used to be available) that was much thicker than the current Nutella that is available in the US.
1/2 cup Nutella
1/4 to 1/3 cup heavy cream
Cook in heavy-bottomed sauce pan over MEDIUM heat*, stirring constantly until the mixture is completely smooth (~7-10 minutes). Let ganache cool slightly (~5 minutes). Pour over warm Nutella Bundt Cake (or any kind of cake). Let the ganache cool completely before serving.
*Do NOT use HIGH heat; if you rush this, you won't like the results.
Note: If the mixture is too thin to set up as a typical ganache, simply re-warm it and stir in a handful of chocolate chips (preferably semi-sweet) until smooth. Also, I have not tried to whip this frosting, so I do not know if it can be used as a substitute for whipped ganache.