1. Combine Nutella, chocolate, and condensed milk in a large glass mixing bowl. Microwave for about 30 seconds and then stir to help everything combine. Microwave for about another 30 seconds and mix again until chocolate is completely melted and mixed. You may need to microwave slightly longer, but make sure not to overcook the chocolate.
2. Line an 8 x 8 inch pan with parchment paper. Pour fudge mixture in and attempt to even out top and smooth with hand. If using, sprinkle hazelnuts across surface and press down slightly into the fudge. Refrigerate for an hour or more to set. Remove and slice. Keep uneaten portions in fridge to keep from getting slightly melty/sticky. Due to the Nutella oils, this will get sticky and oily at room temperature.