Preheat the oven to 180c/350f and line a deep 20cmx30cm baking tin with baking parchment. Leave a little of the parchment overhanging the tray to make it easier to get the brownie out later.
Melt the dark chocolate and butter together in the microwave in 30 second bursts, giving it a stir each 30 seconds (or you can melt in a bowl over a pan of simmering water - make sure the bowl isn't touching the water though). Stir and leave to cool slightly.
Whisk the eggs and sugar in a large bowl using an electric whisk until thick and creamy.
Slowly whisk again, and pour the melted chocolate and butter in. Pour it at the side of the bowl in a thin stream, whisking all the time, so that the eggs don't scramble with the warm chocolate.
When the chocolate is thoroughly mixed in, sieve the flour, cocoa powder and baking powder over the mixture, and then add the ground almonds. Fold into the mix gently, in a figure of eight. This will help to keep the air in.
Pour the mixture into your baking tray, and use a palette knife to ensure it goes all the way to the edges.
Arrange the Ferrero Rochers on top of the brownie mix evenly - so that you'll remember where to slice the brownies once they're cooked. Push the Ferrero Rochers into the mixture slightly and place the tray in the oven.
Cook for 25 minutes.
Take out of the oven and leave to cool in the tray. Lift the brownie out of the tray using the baking parchment and place on a chopping board.
Whilst the brownies are cooling, make the ganache. Heat the cream in a saucepan until it's just about to come to the boil, then turn the heat off. Add the chocolate, stir once, and place a lid on the pan. Leave for 10 minutes for the chocolate to melt. After 10 minutes, stir the ganache until it comes together in a smooth mixture. Leave to cool for a further 10-20 minutes, so it thickens to a consistency where it's pour-able, but thick enough not to run straight off the brownies.
Pour the ganache all over the top of the brownie, and spread to the edges if required using a spatula. Sprinkle the chopped nuts on top.
The ganache won't set completely - it's supposed to be thick and sticky. It will firm up a little if you leave it for an hour or so.
Use a sharp knife to slice the brownie into 12 portions - try to ensure there's a full Ferrero Rocher in each portion.
Notes
Leftover brownies can be covered and refrigerated for up to 2 days..Nutritional Information is per serving.