Melted butter, to grease
1 ½ cup roasted hazelnuts
1/2 cup granita biscuits, finely crushed
50g butter, melted
2 Tbsp vegetable oil
200g 70% cocoa chocolate, chopped
200g dark cooking chocolate, chopped
220g chocolate-hazelnut spread
1/2 cup sugar
1 cup thickened cream
Thickened cream (optional)
Unsweetened cocoa powder (optional)
Toasted hazelnuts (optional)
Preheat oven to 180C. Grease base and sides of a 25cm springform tin. Wrap outside tightly with heavy foil; set aside.
Crush 1/2 cup of the hazelnuts and combine with biscuits and butter in a small bowl. Toss gently. Press mixture evenly onto base of prepared tin.
Pulse remaining hazelnuts in a food processor until a paste forms, add oil and process until smooth. Set aside. In a large saucepan, cook and stir both chocolates over low heat until melted and smooth. Remove from the heat. Stir in hazelnut butter and chocolate-hazelnut spread.
Beat eggs in the extra-large bowl of an electric mixer on medium speed until frothy. Gradually add sugar, beating on medium-high speed until mixture is thick and pale and holds a slow dissolved ribbon when beaters are lifted (about 5 minutes in a heavy-duty mixer or 9 minutes with a handheld mixer.) Pour chocolate mixture into egg mixture, stirring gently to combine.
Beat thickened cream in a medium bowl on a medium speed until soft peaks form. Gently fold whipped cream into chocolate mixture. Pour chocolate mixture onto crust, spreading evenly.
Put tin in a shallow roasting tin and pour in enough hot water to reach halfway up sides of springform tin. Bake for 1 ¼ hours. Turn off oven; let cake stand in oven with door closed for 45 minutes.