Prheat oven to 300F. Spread hazelnuts on a rimmed baking pan and roast, stirring once or twice, until fragrant and lightly browned, about 12 minutes. Transfer to a rack
Split vanilla pod lengthwise and scrape the seeds into a saucepan. Add vanilla pod, milk, cream, 1/4 cup sugar, and salt. Simmer over medium-low heat, stirring occasionally, for 8 minutes
Meanwhile, rub still-warm hazelnuts in a clean dish towel to remove skins. Take saucepan off heat and allow cream to cool while you prepare hazelnut paste
In a food processor, place 1/2 cup hazelnuts, remaining 1/4 cup sugar, and the Cognac or rum; puree until paste forms, about 3-5 minutes. Scrape paste into saucepan with milk mixture and return it to medium-low heat. Simmer, stirring occasionally, until paste dissolves, about 5-7 minutes. Discard vanilla pod and stir in vanilla extract. Transfer to a bowl and place in refrigerator to cool
Coarsely chop remaining hazelnuts
When mixture is cold, freeze in ice cream maker according to manufacturer directions along with 1/4 cup chopped hazelnuts. Transfer to container and freeze at least 2 hours, until solid. Serve with remaining hazelnuts