Lightly grease and flour a 9-inch/26cm springform pan. Line the bottom with waxed or parchment paper; grease paper.  Place hazelnuts in a sheet pan. Bake 10 to 15 minutes or until toasted and fragrant, shaking pan occasionally. Remove pan from oven. Wrap hot hazelnuts in clean cloth towel roll hazelnuts back and forth to remove the skins. In food processor, place 3/4 cup/90g nuts and 1/4 cup/50g sugar; pulse until finely ground. Roughly chop remaining nuts; set aside separately.

Melt chocolate and butter on medium-low, stirring occasionally. Meanwhile, in large bowl, with mixer on medium-high speed, beat eggs and 1/2 cup sugar/180g  7 minutes or until tripled in volume. With rubber spatula, fold in chocolate mixture, then fold in ground-nut mixture. Pour batter into prepared pan and bake 35 minutes or until top is dry and cracked and toothpick inserted in center comes out slightly wet. Cool in pan on wire rack 10 minutes. Remove side of pan and cool 30 minutes longer on wire rack.  Line 9-inch/26cm pan with foil. Spread reserved chopped hazelnuts in single layer. In a skillet, spread remaining  sugar in even layer. Cook on medium-high 3 to 5 minutes or until sugar is melted and golden amber, swirling sugar in pan to cook evenly. (Do not stir.) Drizzle melted sugar over nuts to coat evenly. Cool praline completely in pan. When cool and hardened, break into 12 large pieces. In a large bowl, beat cream until soft peaks form, 3 to 5 minutes.

Cut cake into 12 pieces Top each slice with whipped cream and shard of hazelnut praline.