Place half of your toasted hazelnuts in a mini food processor and pulse a few times until well chopped. Combine with heavy cream in a sauce pot and bring to a low boil. Reduce heat and let simmer 30 minutes. Remove from heat and let cool slightly.
In a large bowl, whisk egg and egg yolks until pale and well combined, about 2 minutes. Slowly whisk in sugar until fully combined, then whisk an additional 30-60 seconds more.
Add cold milk to the bowl and whisk well. Then, slowly and gradually add in the heavy cream mixture, whisking constantly to prevent the eggs from scrambling. Once about half of it has been added slowly you can add the rest more quickly. Whisk well, transfer to an airtight container and place in the fridge for at least 12 hours to chill.
The next day, strain the mixture through a fine wire mesh strainer to remove the hazelnut pieces.
Churn immediately according to your ice cream maker's instructions. The ice cream will likely still be quite soft once it's done churning. Transfer back to the container, press a sheet of plastic wrap flat over the surface of the ice cream to prevent freezer burn, and chill for at least 1-2 hours before stirring in the hazelnut praline pieces.