Whisk the egg whites and salt until stiff but not dry. In a separate bowl, beat the butter and Nutella. Mix in the Frangelico (or rum or water), egg yolks, and ground hazelnuts. Fold in the melted chocolate. Then, fold in the egg whites.

Pour the batter into the prepared pan. Bake for 40 minutes, until the cake begins to come away from the sides of the pan. Cool the cake completely on a wire rack.

Toast the hazelnuts in a skillet until golden brown in places. Transfer to a plate to cool. (Be sure to cool completely before putting them on the cake.)

In a heavy saucepan, heat the cream, Frangelico (or rum or water), and chocolate. When the chocolate has melted, remove the pan from the heat and whisk until thick enough to frost the cake.

Remove the sides from the pan. (You can leave it on its base or carefully transfer it to another serving plate.)

Frost the top of the cake with the frosting. Top with the cooled toasted hazelnuts.