Great question — answering as the UphillTech team:
Using a confectionery emulsifier , which reduces oil separation might help. Failing that, instead of using Nutella, add hazelnut butter into the ganache. If the nut-butter has no emulsifiers, it will separate on its own - so some of the (delicious) excess oil could be removed before use. It's also possible to buy nut powders, but these may cause textural issues.
Full answer by Kingsley: https://cooking.stackexchange.com/a/125727 (CC BY-SA 4.0).
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