Place milk chocolate and dark chocolate into a medium to large sized heat safe bowl. Set aside.
In a small sauce pan on stove top over medium heat, stir together heavy whipping cream, butter, corn syrup and salt. Bring to a boil, stirring occasionally. Once boiling, pour mixture over chocolate chips. Cover bowl with plastic wrap and let sit for 5 minutes.
Remove plastic wrap and whisk until smooth. Then whisk Nutella into chocolate mixture until smooth.
Place bowl in refrigerator for 15 minutes, then stir well. Place bowl back in refrigerator for another 15 minutes, then stir well.
Again, place bowl back in refrigerator, but stir after 5 minutes. Repeat the cooling, then stirring, every 5 minutes until chocolate is firm enough to form into balls. This will take 10-15 minutes total.
Once firm, form chocolate into about 1" round balls. Place on parchment paper or silicone mat. Put formed balls in refrigerator for at least 20 minutes (you may can keep the balls in the fridge for up to 24 hours, but if you do this make sure you put them in a container that seals, or cover them in plastic).