Prepare a large tray with a Silpat (silicone mat) or parchment paper, and prepare a mixture of ¾ cup (210 g) room-temperature Nutella with 3 tbsp (24 g) chopped hazelnuts. Spread it in the Silpat, making a square smaller than 8 inches (20 cm).

For the top decoration, mix 9 tbsp, ½ cup (144 g) Nutella with 1 tbsp (8 g) crushed hazelnuts. On the other side of the Silpat, prepare 9 round pieces of this Nutella mixture; this will be what is on top to decorate; it doesn’t have to be perfect. Freeze for 30-45 minutes.

For the brownie: in a stand mixer bowl, crack 4 room-temperature eggs, add ½ cup (122 g) of granulated sugar and ½ cup (104 g) light-brown sugar. Using the whisk attachment, whisk the eggs first on medium-low speed for 2-3 minutes (speed 2) and then increase the speed to maximum. Whip until the sugar has dissolved and the eggs have turned pale. Set aside.

Brown the butter: In a light-colored large pan, melt 1 cup (200 g) unsalted room temperature butter, stirring constantly. It will go from melted to bubbly, more intense bubbling, and finally, it will start to brown. Once it reaches a nice golden-brown color, turn off the stove. Add to the hot butter ¾ cup (150 g) dark chocolate callets and let it sit for 1 minute, then stir until it’s all fully melted, add ¾ cup (150 g) room-temperature Nutella and mix well.

In a large mixing bowl, add ¾ cup minus 1 tbsp (100 g) all-purpose flour, ½ cup + 1 tbsp (70 g) cocoa powder, 100%, and ½ tsp (3 g) fine sea salt. Mix it all very well. To this, add the melted chocolate mixture and mix again until there are no more lumps. Add the whipped eggs and fold them with a spatula, being very careful not to deflate them.

Preheat the oven to 355F (180 °C). Line the square 8-inch (20 cm) pan with two lines of parchment paper, so it covers all 4 sides for easy lifting. Add half of the brownie batter into the square pan and, using an offset spatula, spread it to cover all the edges. Place the frozen square Nutella we prepared earlier in the middle and cover with more brownie batter, smooth it out.

Place the 9 Nutella decoratives we prepared earlier on top and bake for 35 minutes.

Once they come out of the oven, let them cool for 15 minutes and place the Ferrero Rocher on top of each one of the Nutella rounds. Let the brownies cool in the pan for 2 hours before attempting to remove them from the pan.

You can easily remove them by lifting the parchment paper from the sides. Slice into 9 squares and serve.