Preheat the oven to 180ºC or 350ºF.
Melt the butter and dark chocolate over a double boiler/ Bain-Marie/ in the microwave.
Mix the eggs with the caster sugar and vanilla extract.
Add the chocolate mixture to the egg mixture and mix well.
Sift in the dry ingredients into the wet mixture and gently stir to combine.
Chop up 8 of the Oreo cookies into small bits and stir them into the batter.
Pour the batter into a greased 8 x 8 inch pan (lined with parchment paper).
Cut the remaining cookies into large chunks and press them on top of the brownie batter.
Bake in the oven for around 25 to 30 mins until the edges are firm but the centre is only slightly set. Allow the brownies to cool completely before cutting.