Preheat oven to 350°F.

Line a 9×9 baking pan with parchment paper, or spray with non-stick spray.

Combine the melted butter, brown sugar, white sugar, and vanilla in a large bowl. Using a hand mixer, on low speed, mix for 1-2 minutes until well combined and the sugar crystals have dissolved.

Add the eggs and mix on low speed, just until the ingredients are combined.

Whisk the flour, baking powder, and salt together, then add to the bowl. Stir just until combined.

Gently crush the Ferrero Rocher balls with the back of a spoon, then fold half of them into the batter, leaving 1/2 aside to sprinkle on the top.

Spread the batter into the prepared 9×9 pan, then dollop the Nutella on top.

Swirl the Nutella into the batter with a knife, then sprinkle the remaining crushed chocolate on top.

Bake for 25-30 minutes until golden brown, the top feels set, and a toothpick inserted in the center comes out mostly clean.

Move the pan to a cooling rack. Once cool, if you have used parchment, lift the bars out of the pan to slice. Otherwise, you can cut them in the pan and remove them.

Store in an airtight container at room temperature for up to a week, or in the freezer for several months.