Better Than Nutella (Chocolate-Hazelnut Spread)

Preheat oven to 350 °F. Spread out nuts on a rimmed baking sheet or in an ovenproof skillet. Roast, shaking sheet once for even toasting, until deep brown, 13-15 minutes. Let cool completely. (If nuts have skins, rub them in a kitchen towel to remove.) Grind hazelnuts and sugar in a food processor until a fairly smooth, buttery paste forms, about 1 minute. Place chocolate in a medium metal bowl. Set bowl over a large saucepan of simmering water; stir often until chocolate is melted and smooth. Remove bowl from over saucepan; add butter and whisk until completely incorporated. Whisk in cream and salt, then hazelnut paste. Pour gianduja into jars, dividing equally. Let cool. (Gianduja will thicken and become soft and peanut butter- like as it cools.) Screw on lids. DO AHEAD: Gianduja can be made up to 4 weeks ahead; keep chilled. Let stand at room temperature for 4 hours to soften. Can stand at room temperature up to 4 days.