Get your oven ready by preheating it to 200C/400F/Gas 6.
Place hazelnuts on a baking tray and roast in the center of the oven for 10 minutes. Remember to shake the pan halfway through to ensure even roasting until they turn a beautiful golden brown. Keep a close eye on them to prevent burning. Afterward, let them cool for a few minutes.
After adjusting the oven temperature to 180C/350F/Gas 4, prepare a 23cm/9in springform cake tin by greasing it and lining the base with baking parchment for your convenience.
In a food processor, finely grind 200g/7oz of hazelnuts, ensuring they resemble fine breadcrumbs with a hint of texture. Reserve the remaining 50g/2oz hazelnuts for later use.
To prepare, put butter and chocolate in a heatproof bowl. Position it over a pan of gently simmering water. Stir occasionally, and when almost all the chocolate is melted, take the bowl off the pan. Allow the residual heat to complete the melting process.
Incorporate the ground hazelnuts, then allow the mixture to cool for 5 minutes.
Begin by whisking egg yolks and sugar together using an electric hand-whisk for a minimum of 5 minutes until you achieve a pale and creamy consistency. Then, blend this mixture thoroughly into the chocolate. Don't forget to wash and dry the beaters afterwards.
Start by whisking egg whites in a clean bowl until stiff peaks form. You'll know they're ready when you can confidently flip the bowl upside down without the eggs spilling out. Remember, don't over-whisk.
Stir the hazelnut liqueur, amaretto, or orange juice into the chocolate mixture quickly to soften it. Next, incorporate a couple of tablespoons of the whisked egg whites and stir until well combined. Finally, gently fold in the remaining egg whites.
Pour the mixture into the prepared tin, then place it in the center of the oven. Bake for 35-40 minutes until the cake rises and becomes firm.
After removing the cake from the oven, allow it to cool in the tin for 30 minutes. Next, release the springclip, flip the cake onto a serving plate, and gently peel off the lining paper. Let it cool completely.
After refrigerating, generously apply the chocolate-hazelnut spread onto the cake and garnish with the hazelnuts you saved. They can be whole or finely chopped, depending on your preference.