Flick oven onto 200°c bake and line a square 20cm tin with baking paper.
Melt the butter in a medium sized saucepan, swirling until melted. Then take off the heat to cool.
While butter is cooling, sift flour, baking powder, cocoa powder and salt in a large bowl. Then add chopped chocolate and broken Oreo pieces and set aside.
When butter has cooled slightly, add sugar, vanilla and eggs and mix until smooth.
Fold wet ingredients in to the dry until just combined. Don't over mix.
Pour mixture into prepared baking tin and bake for 18-20 minutes or until top has set. Then place the tin straight into the fridge to stop it baking further.
When the brownie has cooled you can start on the cookies and cream icing by beating the cream cheese either in a free-standing mixer or with a hand-held beater until smooth.
Add the vanilla and icing sugar in stages until the icing starts to come together. Finally crush the remaining Oreo cookies and fold through the icing with a spatular.
Spread over top of cooled brownie base and return to the fridge to set for about an hour. Cut and serve with a glass of milk!