Preheat oven to 350 degrees F. Butter a 9-inch springform pan. Line bottom of pan with parchment paper. Wrap outside of pan tightly with heavy foil; set aside.
In a large mixing bowl beat eggs, brown sugar, and 1 tablespoon of the Frangelico with an electric mixer on medium to high speed for 3 to 4 minutes or until pale and thickened. Reserve 1 tablespoon of the chocolate-hazelnut spread. In a medium bowl combine the remaining chocolate-hazelnut spread and melted butter. Stir butter mixture into egg mixture. Stir in ground hazelnuts, melted chocolate, cocoa powder, and salt. Transfer batter to the prepared springform pan, spreading evenly.
Place springform pan in a shallow roasting pan. Place roasting pan on oven rack. Pour enough hot water into the roasting pan to reach halfway up sides of springform pan. Bake for 1 1/4 hours (center should be set and top dry to the touch).
Carefully remove springform pan from water; remove foil from pan. Cool cake in pan on a wire rack for 15 minutes. Loosen cake from sides of pan; cool for 45 minutes more. Remove sides of pan. Cover and chill for at least 3 hours before serving.
In a medium mixing bowl beat 1/2 cup whipping cream, granulated sugar, and the remaining 1 tablespoon Frangelico on medium speed until soft peaks form (tips curl). Spread whipped cream over top of cake and, if desired, sprinkle with chopped hazelnuts. In a small bowl whisk together the reserved 1 tablespoon chocolate-hazelnut spread and 2 teaspoons whipping cream; drizzle over cake.