Crush the cornflakes with melted butter and cocoa powder.
Cover the bottom of a springform pan with the crushed cornflakes.
Refrigerate for 20 minutes.
Toast the hazelnuts in the oven.
Blend the still-warm hazelnuts in a food processor to form a paste.
Add half of the cream, cream cheese, vanilla extract, and sugar.
Mix until all ingredients are incorporated.
Pour hazelnut mixture onto the crushed cornflakes in the springform pan, then spread with a spatula.
Refrigerate until firm.
Melt the chocolate with the remaining cream.
Cover the cheesecake with chocolate.
Decorate by topping with Ferrero Rocher chocolates.
Let the cheesecake cool for 30 minutes.
Slice and serve.