Preheat oven at 170°C or 338°F. Line a 5cm deep square pan with parchment paper. Set aside.
Using a heatproof bowl, mix chocolate melts and butter. Place the bowl over a pot with simmering water. Make sure the bowl is not touching the water. Stir until everything is melted and well combined. Remove from the heat and let it cool for at least 5 minutes.
Add the eggs, brown sugar and vanilla to the chocolate mixture. Sift the flour, salt and cocoa powder over the mixture and mix well.
Pour the brownie mixture into the pan and put the crushed peanut on top. Bake at 170°C or 338°F for 30 minutes. Set aside to cool before slicing into 12 equal pieces. Store in an air-tight container and put in the fridge for up to one week or at room temperature for up to 3-4 days.