Preheat your oven to 350 and grease a 9*13 pan. You can also use a 8*8 square tin and a 5*7.
Melt the butter over low heat, then pull the pan off the stove and stir in the chopped chocolate until it melts
Beat the eggs, sugar, salt and vanilla on high for 4-5 minutes until the mix becomes pale and thick. When you lift the beaters out the batter should form a wide ribbon that folds back on itself and slowly dissolves on the surface.
With a light hand, fold the cooled chocolate into the eggs, followed by the flour.
Line up your Ferrero Rocher nice and pretty.
Take a little batter and cover the top of the chocolate balls to protect the chocolate from burning.
Bake until the top looks slightly cracked 25-35 minutes (depending on pan)
Cool completely before cutting into squares
There is no leavening agent in this recipe so the rise of the brownies comes from beating the eggs. If you use cold eggs they will not attain the kind of volume you are looking for. If your eggs are cold then fill a bowl with hot water and let the eggs sit in it for a few minutes until they are lukewarm to touch.