Heat the oven to 170 C / 340 F. Butter a 20 cm / 8-inch springform pan and line the base with parchment.

Toast the hazelnuts on a sheet pan for 8 to 10 minutes until fragrant, then rub in a clean towel to remove most of the skins. Let cool, then pulse in a food processor to a fine, sandy meal.

Melt the chocolate and butter together in a heatproof bowl set over a pan of barely simmering water, stirring until smooth. Remove from heat and cool slightly.

Whisk the egg yolks with 100 g of the sugar until pale and thick, about 2 minutes. Stir in the melted chocolate mixture, then fold in the ground hazelnuts and salt.

In a separate clean bowl, whip the egg whites to soft peaks, then add the remaining 50 g sugar and whip to stiff, glossy peaks.

Fold a third of the whipped whites into the chocolate batter to loosen it, then fold in the rest gently in two batches until no white streaks remain.

Pour the batter into the prepared pan and smooth the top. Bake for 35 to 40 minutes, until the top is set and slightly cracked and a skewer inserted in the center comes out with a few moist crumbs.

Cool in the pan for 10 minutes, then run a knife around the edge and release the springform. Cool completely on a rack before glazing.

Melt the hazelnut-chocolate spread with the cream in a small saucepan over low heat until smooth. Pour over the cooled cake and tilt the pan to spread it evenly.

Scatter the chopped toasted hazelnuts over the glaze before it sets. Slice and serve at room temperature.