Melt 150g dark couverture chocolate using a double boiler. Allow it to cool naturally.

Melt 150g room temperature butter using a double boiler. Remove once fully melted.

Mix the melted butter with 160g brown sugar until smooth.

Add 3 room temperature eggs one at a time, mixing thoroughly after each addition.

Add a dash of vanilla extract to remove any eggy smell.

Pour the cooled melted chocolate into the mixture and mix well.

Sift 40g cake flour, 20g cocoa powder, and 2g baking powder into the mixture. Mix until smooth.

Add 50g of whole hazelnuts into the batter and mix evenly.

Line a square baking tin with parchment paper and pour the batter in. Smooth the surface and tap the tin on the counter to remove air bubbles.

Top the batter with 10g of hazelnuts, pressing them gently into the surface. Bake at 160°C for 25 minutes.