Preheat the oven to 180C / 350F / 160 fan forced. Line an 8x8 inch / 20x20cm brownie tray with baking paper (if you crinkle it up first it will fit into the corners better)
Melt together the butter and chocolate in the microwave in 30 second bursts, stirring well between each until just melted. Allow to cool while you get everything else together.
Mix the flour, baking powder, cocoa and sugar together, making sure there are no lumps. Mix well so everything is distributed. Make a well in the centre and add the butter, chocolate and vanilla. Stir to distribute a little, then add the beaten eggs and stir the whole lot until just combined.
Tip half of the batter into the prepared pan and use a spoon or spatula to spread to all the edges and into the corners. Drop the Ferrero Rochers into the pan randomly. Use a spoon to dollop the rest of the batter over and in between the Ferreros making sure to get into all the corners.
Bake for 10 minutes, then remove and dollop the Nutella
Bake for a further 15-20 minutes until firm around the edges but the middle should still be a little soft (with a very slight wobble if you shake the pan).
Allow to cool well before slicing