Preheat oven to 350℉. Spread hazelnuts in a single layer on baking sheet. Bake for 10-15 minutes, stirring midway through baking, until the skins begin to crack and the nuts start to brown.
Transfer roasted hazelnuts into a clean kitchen towel. Use the towel to vigorously rub the hazelnuts to remove the skins. Some of the skin may not want to peel. Simply remove as much as you can. Discard skins. Cool hazelnuts while you prepare the ice cream base. Chop cooled hazelnuts and set aside.
In a medium pot, combine cream and milk. Set over medium heat and bring to a simmer. Stir as necessary to ensure milk does not burn along the sides or bottom of the pot.
Meanwhile, in a large bowl, whisk together egg yolks and maple syrup. Once milk mixture has almost come to a boil, lower heat. Temper hot milk into sugar-yolk mixture. Slowly add hot milk into yolks about ¼ cup at at time, whisking to distribute heat. Once all the milk has been added to yolk, transfer all contents back into the pot.
Stir over medium-low heat until mixture reaches nappe consistency, or 180℉ on a kitchen thermometer. Stir frequently to ensure mixture does not burn at the bottom. Let mixture sit at 180℉ for 2 minutes to pasteurize eggs. Remove from heat and add sea salt. Stir to combine.
Transfer mixture to an airtight container and let mature in the fridge overnight. Freeze mixture in ice cream maker according to your unit's instructions. Once the volume has increased by half and has reached a soft serve consistency, add in chopped hazelnuts. Transfer ice cream to an airtight container and let chill in the freezer for at least 30 minutes before serving.