Toast whole hazelnuts in a dry 10-inch skillet over medium heat for 4 to 5 minutes, tossing often, until fragrant and the skins crack and blister.

Rub the warm hazelnuts in a clean kitchen towel to remove most of the skins. Let cool completely.

Pulse 100 g of the cooled hazelnuts in a food processor until finely ground, almost like coarse sand. Finely chop the remaining 50 g by hand and set aside for coating.

Heat the heavy cream in a 1-quart saucepan over medium-low heat until steaming, about 80 C / 175 F, 3 to 4 minutes. Do not let it boil.

Remove from heat, add the chopped sugar-free chocolate, and let it sit for 2 minutes to soften.

Whisk until the ganache is smooth and glossy. Whisk in the butter, erythritol, vanilla, and salt until fully melted and combined.

Fold in the ground hazelnuts until evenly distributed through the ganache.

Cover the bowl and chill for 2 hours, until the ganache holds a dent when pressed with a spoon and no longer looks glossy and loose.

Scoop tablespoon portions, about 25 to 27 g each, and roll into balls between your palms, working in small batches so the mix stays cold.

Roll each ball in the chopped hazelnuts or cocoa powder to coat, then place on a parchment-lined sheet pan.

Chill the coated truffles for 20 minutes more before serving, or transfer to a container for storage.