Unwrap the chocolates, place on a tray and pop in the freezer. Heat oven to 180C/160C fan/gas 4. Lightly grease and line the base and sides of a 21-22cm square tin with baking parchment.
Put the butter, sugars and cocoa into your largest saucepan and gently melt together, stirring regularly so the mixture doesn’t catch. Once the sugar granules have just about disappeared, take off the heat, tip into a bowl and leave to cool for 5 mins.
Use a whisk or wooden spoon to 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, hazelnuts and liqueur. Tip the mixture into the prepared tin and bake for 35 mins.
Remove the tin from the oven and use a cutlery knife to mark the top of the brownies into 16 squares (don’t cut through, it’s just as a guide). Use a teaspoon to push a little dent in the centre of each portion and add a frozen Ferrero Rocher chocolate into each dip. Return to the oven for 3 mins, then remove and leave to cool completely.
Once cool, cut into 16 squares. Will keep for 3 days in an airtight container.