Preheat the oven to 200°C.
Roast hazelnuts on a tray for 10 minutes until golden brown.
Reduce oven temperature to 180°C and prepare a 23cm springform tin with butter and parchment.
Grind 200g of hazelnuts finely in a processor and reserve the remaining 50g.
Melt butter and chocolate together in a bowl over simmering water.
Stir ground hazelnuts into the chocolate and cool for 5 minutes.
Whisk egg yolks and sugar for 5 minutes until creamy and combine with chocolate.
Whisk egg whites in a separate bowl until stiff peaks form.
Stir liqueur into the chocolate and gently fold in the whisked egg whites.
Bake in the tin for 35 to 40 minutes until firm.
Cool in the tin for 30 minutes before inverting onto a plate to cool fully.
Spread with hazelnut chocolate spread and sprinkle with reserved hazelnuts.