Preheat the oven to 180°C or 350°F. Spray a 9 inch circle pan with oil and cover with parchment paper. Set aside.
Melt the butter and the chocolate together either in the microwave or on bain-marie.
In a large bowl mix together the sugar, brown sugar, salt, eggs and vanilla extract.
Combine together the wet mixture with the melted butter and chocolate.
Sift in the flour and cacao powder and mix well together. Finally add the chopped hazelnuts and stir them in.
Transfer the batter in the pan. Cook the brownies for 25 minutes.
Once ready, let the brownies rest completely for about an hour. It's important that the brownie is completely set before decorating.