Spread about half a jar of Nutella onto baking paper in the shape of your 20x20 cm pan. Freeze it until firm so it can be placed into the brownie easily.

Melt the butter and milk chocolate together in the microwave or over a water bath until smooth. Stir in the cocoa powder and let it cool slightly.

In a large bowl, whisk the eggs with the white sugar and brown sugar until the mixture looks light and slightly fluffy. Add vanilla extract and a pinch of salt.

Pour the melted chocolate mixture into the egg mixture and stir to combine. Then fold in the flour and baking powder gently until no dry spots remain.

Spread half of the brownie batter into a lined 20x20 cm pan. Place the frozen Nutella layer on top, then cover carefully with the remaining brownie batter.

Bake at 170°C for 28–30 minutes, or until the brownie is set but still fudgy in the center.

Let the brownie cool completely before slicing so the layers stay neat.