Preheat oven to 350°. Spread out nuts on a rimmed baking sheet or in an ovenproof skillet. Roast, shaking sheet once for even toasting, until golden brown, 13-15 minutes. Let cool completely. (If nuts have skins, rub them in a kitchen towel to remove, see photos in post for tips.)

Grind hazelnuts and sugar in a food processor until a fairly smooth, buttery paste forms, about 1 minute. You can use granulated sugar, or I prefer powdered sugar for a creamier consistency.

Place chocolate in a medium metal or glass 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, then hazelnut paste.

Pour mixture into jars, dividing equally. Let cool (you can pop it in the fridge to speed up cooling). Mixture will thicken and become soft and peanut butter-like as it cools. Screw on lids.