Preheat oven to 200C/180C Fan/Gas 6. 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. Set aside to cool whilst making the brownies.
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 mix again. Then check that the dark chocolate has cooled a little before adding in and combining. Add in the flour, cocoa powder and chopped Oreos and fold together. Pour into the tin.
Make the cheesecake layer by combining all ingredients in a small bowl and mixing with a spoon. Pour this over the brownies. Swirl using a knife or chopstick. Add remaining Oreos - chopped/crushed.
Bake 30-35 minutes. Let cool before slicing.
