• Preheat oven to 370℉/180℃. Line a 8″ square baking tin with greaseproof paper. 
• Melt the dark chocolate by breaking up into small pieces in a bowl and heating for 30 second intervals in a microwave, stirring in between. Alternatively, set up a double boiler. Do this by placing a heatproof bowl over a small pot of simmering water. Add the chocolate to the heatproof bowl and stir until melted. Set aside. 
• In a large bowl or the bowl of a stand mixer combine the melted butter, and both the brown and granulated sugars on a medium speed until totally combined.
• Add the eggs and vanilla extract and mix again. Add in the flour and cocoa powder and fold together.
• Pour into the tin. Bake for 30 minutes. Remove and let cool.

For the Ganache

• Chop the chocolate to fairly small pieces - this doesn't need to be too even, just think of a 'fine chop'. Put into a heatproof bowl.
• Heat the cream in a small saucepan on low. Keep stirring until the cream comes to the boil. Immediately turn off the heat and pour the cream into the bowl with the chocolate. Let rest 1 minute before stirring until the mixture comes together to form a glossy/rich consistency. Let rest 5 minutes then pour and spread over brownies.
• Put brownies in fridge to firm up for about 45 minutes, then slice and serve!