Beat the super soft or partially melted butter, brown sugar and sugar together.
Beat in the eggs one at a time.
Add the vanilla extract and golden syrup and beat in.
Mix in the flour and baking powder.
Stir in the chocolate chips.
Set this aside.
Brownie Layer;
Melt the butter and chocolate together.
Beat the eggs, sugar and vanilla extract until light and fluffy.
Mix the chocolate mixture through the eggs.
Mix in the cocoa, followed by the flour.
Line a 9"x13" baking tin with cooking paper and preheat the oven to 180°C (350°F).
Spread the cookie dough over the base of the baking tin.
Top with the Oreos.
Spread over the brownie batter evenly.
Place in the oven for 35-45 minutes. The middle will slightly wobble and a toothpick inserted should have loose crumbs attached. The sides and top should be set.
Allow to cool before slicing and serving.
Notes
You can halve the cookie and brownie layers for a flatter brownie.