Preheated the oven to 160Â°C fan/180Â°C.
In a bowl over a pan of simmering water, melt together the butter and dark chocolate.
Once melted, remove from the heat and sieve in the cocoa powder and plain flour.
In a seperate bowl, with an electric whisk, whisk together the eggs and caster sugar for 7-8 minutes until it double in size and becomes light and pale.
Pour a little bit of the melted chocolate and butter into the egg mixture and fold through. Repeat this until all of the chocolate mixture is fold through the egg mixture.
Add in the white chocolate chunks and broken golden oreo pieces. You may want to leave a few biscuits left to place on top of the brownies.
Pour the mixture into a 8" inch lined square tin and bake in the preheated oven for 25 minutes
Once cooked, allow the brownies to cool in the tin fully before sliceing in to 9 squares.