Heat the oven to 180°C. Lightly grease and line the base and sides of a tin with baking parchment. Put the butter, sugars and cocoa into a saucepan and gently melt together, stirring regularly. Once the sugar granules have just about disappeared, take off the heat, tip into a bowl and leave to cool. Beat the eggs, one by one, into the mixture. When they’re completely incorporated and the mixture is smooth and shiny, stir in the flour and hazelnuts. Tip the mixture into the prepared tin and bake for 35 mins. Remove the tin from the oven and mark the top of the brownies into squares with a knife (don’t cut through). Add a frozen Ferrero Rocher chocolate into each square. Return to the oven for three minutes, then remove and leave to cool completely.