In a medium saucepan, combine whole milk, heavy cream, and half of the sugar. Heat over medium heat, stirring gently until the mixture is steaming but not boiling.

In a separate bowl, whisk together the egg yolks with the remaining sugar until the mixture turns pale and thickens, about 1 minute.

Gradually pour 1 cup of the hot milk mixture into the whisked egg yolks while constantly whisking to prevent the yolks from scrambling. Return the mixture back into the saucepan.

Cook the combined mixture over low heat, stirring constantly until it thickens enough to coat the back of the spoon, around 5-10 minutes.

Remove the saucepan from heat and whisk in the Nutella, cocoa powder, and vanilla extract until completely smooth.

Pour the creamy mixture through a fine-mesh sieve into a bowl to catch any bits of cooked egg.

Allow the mixture to cool to room temperature, then cover with plastic wrap and refrigerate for at least 4 hours, or overnight.

Churn the mixture in an ice cream maker until it reaches a soft-serve consistency, about 20-25 minutes.

Transfer the churned gelato into an airtight container and freeze for at least 2 hours until firm.

Scoop the gelato into bowls and optionally drizzle with extra Nutella and sprinkle with chopped toasted hazelnuts.