Melt butter in a microwave then add the caster sugar and cocoa powder to the melted butter. Heat in the microwave in 30 second bursts until the sugar has almost dissolved and the ingredients have combined, stirring after each burst.
Allow the mixture to cool slightly before adding the vanilla extract and eggs. Whisk until just combined.
Sift in the flour and gently fold until just combined.
Pour half of the brownie mixture into the prepared tin. Place 12 Oreo cookies into the mixture. Pour over the remaining brownie mixture. Chop the remaining Oreos and press on top of the brownie mixture. Bake for 30 minutes or until cooked through.