• Pre-heat your oven to 180℃ Fan/395℉. Line an 8" Square Pan with greaseproof paper.
• In a small bowl, break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn't burn. 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. 
• In a large mixing bowl, whisk together melted butter and both sugars. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs, one at a time and mixing in between. 
• Now, add the melted chocolate slowly and combine once again. 
• Add the flour, cocoa powder and crushed oreos. Fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Pour into the lined tin. 
• Bake 25-30 minutes until the top is shiny and it's still just a little wobbly in the middle. Let cool.

Make the Frosting

• Whisk the mascarpone or cream cheese with the powdered sugar in a bowl until smooth but thick.
• In another bowl, whisk the cream until soft peaks form. Do this using an electric/stand mixer to save a lot of time and effort. Once soft peaks start to form, add the mascarpone/cream cheese mixture and fold using a spatula or metal spoon. Add the finely crushed Oreos to the frosting and fold in.
• Spread this frosting over the brownie. Add extra crushed Oreos to decorate.