Begin by preheating the oven to 325°F (165C/145C Fan) and lining an 8" square baking pan with parchment paper.
In a large mixing bowl, whisk together the eggs, oil, water and vanilla extract.
Meanwhile in a separate bowl, stir together the dry ingredients - the flour, cocoa powder, powdered sugar, granulated sugar and salt.
Add the dry ingredients to the egg mixture and stir together with a spatula until just combined.
Pour half of the thick batter into the lined baking pan and spread evenly with an offset spatula.
Dollop half of your Nutella over the brownie layer and then use the offset spatula or a kitchen knife to swirl lightly together.
Add the remaining brownie batter on top and spread evenly. Dollop with the remaining Nutella and again swirl through.
Place 9 of your Ferrero Rocher whole into middle of the expected slices of brownies (evenly spaced 3x3 in the square pan).
Chop the remaining 5 Ferrero Rocher into large chunks and sprinkle over the top of the brownie batter.
Place in the centre of the preheated oven and bake for 30-40 minutes until a toothpick inserted about an inch away from the corner comes out mostly clean. (see note).
Remove from the oven and allow to cool for at least 2 hours before slicing. (see note)