Preheat your oven to 160°C (320°F) and line a 20 × 20 cm (8 × 8 inch) square baking tin with parchment paper.
While you prepare the batter, place half of the Ferrero Rocher chocolates in the freezer to make them easier to cut later.
Melt the butter and Nutella together gently in a saucepan over low heat, stirring until smooth. Remove from the heat and allow it to cool slightly.
In a large bowl, crack the eggs and add the caster sugar. Whisk until the mixture becomes pale, thick, and creamy.
Gradually pour the slightly cooled Nutella mixture into the eggs while whisking continuously until fully combined.
Crush the remaining unfrozen Ferrero Rocher and stir them into the brownie mixture until evenly distributed.
In another bowl, sift together the plain flour, cocoa powder, and baking powder.
Gently fold the dry ingredients into the wet mixture until just combined, taking care not to overmix.
Pour the brownie batter into the prepared baking tin and spread it out evenly with a spatula.
Remove the frozen Ferrero Rocher from the freezer and carefully cut them in half.
Press the Ferrero Rocher halves evenly into the surface of the brownie batter.
Sprinkle the chopped hazelnuts over the top, if using.
Melt a little extra Nutella and drizzle it over the top of the batter.
Bake in the preheated oven for about 30 minutes, until a crust forms on top.
For firmer brownies, bake for up to 10 minutes longer, checking carefully to avoid overbaking.
Remove from the oven and allow to cool in the tin before slicing and serving.