Coat the inside of a 9-inch springform pan with non-stick baking spray and line the bottom with a parchment paper circle.
Place the chopped chocolate in a microwave-safe bowl and heat at 30-second intervals on medium power just until melted, stirring after each and being careful not to overheat. Cool slightly.
Add the hazelnuts to a food processor and process them till finely chopped.
Set aside 2 tablespoons of chopped hazelnuts for garnishing the cake.
In a stand mixer or using a hand-held mixer, beat the butter and sugar until light and fluffy.
Gradually add the yolks until fully incorporated.
Mix in the melted and cooled chocolate, chopped hazelnuts, Frangelico, vanilla, and salt.
In a separate mixer bowl with a clean whip or beaters, beat the egg whites until stiff peaks form.
Gently fold ⅓ of the whites into the chocolate hazelnut batter to lighten it.
Carefully fold in the remaining whites until the batter is uniform.
Transfer the batter to the prepared pan.
Bake the cake for 45-50 minutes until the cake springs back lightly when touched.
Cool the cake in the pan for 1 hour, then carefully release the springform side.
Invert the cake onto a serving platter or plate so that the top crust is down. Carefully remove the pan bottom and parchment paper.
To make the glaze, place the chocolate and butter in a microwave-safe bowl and melt together on medium power at 30-second intervals, stirring after each until smooth.
Pour the warm glaze over the top of the cake, allowing some of it to drip over the sides.
Sprinkle the reserved chopped hazelnuts around the edges of the cake.
Refrigerate the cake until the glaze is set (about 45 minutes).
Slice and serve the cake at room temperature. Cover and refrigerate any leftover slices.