Place half of your toasted hazelnuts in a mini food processor and pulse a few times until well chopped. Combine with heavy cream and milk in a sauce pot and heat to just below boiling. (When you tilt the pot you should see lots of bubbles on the bottom.) Reduce heat (or turn heat off) and let steep 30 minutes.
While hazelnuts steep, whisk together egg yolks, egg, and sugar until pale and well combined, 1-2 minutes.
Place the bowl on a folded kitchen towel to anchor in place, then slowly add the heavy cream mixture to the eggs, 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 hours before stirring in the hazelnut praline pieces.