Place the chocolate and butter in a bowl.
In a separate bowl, bring the cream to a light boil and pour over the chocolate. Let it sit for 30 seconds then stir till the chocolate melts completely to create a smooth ganache.
Stir in the nutella - it may be clumpy and difficult to mix at first but a whisk will make it easier to combine. If the mixture looks grainy, reheat it for a few seconds in the microwave untill completely smooth.
Pour the filling into the prepared digestive tart base and gently tap or use the back of a spoon to even out the surface.
Place a clingwrap covering the tart pan and refrigerate it for atleast 1 hour or more.