Spread the hazelnuts on a baking sheet and roast them for 12–15 minutes, until the skins are cracked and the nuts are golden.
Transfer the nuts to a clean kitchen towel and rub vigorously to remove as much skin as possible. Cool completely.
Transfer 2 cups of hazelnuts to a food processor and blend for 8–10 minutes, scraping down the sides frequently, until a smooth butter forms.
Add the melted chocolate, cocoa powder, powdered sugar, oil, vanilla, and salt.
Blend until completely smooth and glossy.
Set aside ½ cup of Nutella for the ice cream base and reserve the rest for swirling.
Whip the cold heavy cream in a large bowl until stiff peaks form.
In a separate bowl, whisk together the condensed milk, cocoa powder, vanilla, and salt until combined.
Gently fold the condensed milk mixture into the whipped cream until no streaks remain.
Fold in the mini chocolate chips, reserved ½ cup of chopped hazelnuts, and reserved ½ cup of homemade Nutella.
Pour half the ice cream mixture into a 9x5-inch loaf pan. Drop spoonfuls of the remaining Nutella over the surface and swirl with a knife. Pour the remaining ice cream mixture on top and swirl again.
Cover tightly with plastic wrap pressed directly onto the surface.
Freeze for at least 6 hours or overnight until firm.
Serve the homemade Nutella chocolate ice cream scooped straight from the pan.