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 the wet mixture with the melted butter and chocolate. Sift in the flour and cocoa powder and mix.
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 assembling the trifle cups.