Cut into tiny pieces and melt the chocolate in a heatproof bowl over a pan of barely simmering water and set aside to cool a little. Beat the butter and icing sugar using an electric hand whisk until the mixture is light and creamy.
Separate yolks from the egg whites and add yolks one at a time to the butter mixture, then add the melted chocolate.
Whip the egg whites with an electric whisk and add them slowly to the chocolate mixture.
Remove 3 tablespoons of the mixture, add the chopped hazelnuts to it, and mix. Pour half of the original chocolate mixture onto an 18cm mould lined with parchment paper, then pour over a layer of the chocolate hazelnut mixture and finish with the remaining chocolate mix. Level the surface with a spatula and place in the freezer for 4 hours.
Soak gelatine in cold water for 10 minutes. Pour water, condensed milk and sugar in a pan until it boils, then set aside. Add chopped white chocolate and the gelatine, squeezing out the moisture. Mix well with an electric mixer, adding red food colouring until you reach the right red colour. Let cool.
Remove from the mould and place on a wire cooling rack, pour over the red glaze and let rest for 10 minutes. Transfer to the serving dish and decorate with four Ferrero Rocher chocolates and redcurrant.
Serve with one Ferrero Rocher per portion.