Preheat the oven to 350 degrees F. Spread the nuts on a baking sheet and roast until deep brown, about 15 minutes. Let cool completely. Rub the nuts in a kitchen towel to remove the skins.

Grind the hazelnuts and sugar in a food processor until a paste forms, about 1 minute.

Place the chocolate in a medium metal bowl. Set the bowl over a large saucepan of simmering water until the chocolate is melted. Remove the bowl from the heat; then add the butter and vanilla and whisk until completely incorporated. Whisk in the cream and salt, then the hazelnut paste.

Pour gianduja into jars, dividing equally. Let cool.