Make the brownie layer: In a large bowl mix the sugar and butter until light and fluffy. Then add the eggs and vanilla essence and mix again. Then sieve in the flour, cocoa powder, baking powder and salt. Mix until fully combined and smooth. Pour this brownie mix into your baking tray and smooth the top with a palette knife.
On top of the brownie lightly press your Jaffa cakes in an even layer. Put this into the freezer for 10 minutes.
Make your cookie layer: In a large bowl beat together your butter and sugar until light and fluffy. Next add your eggs and vanilla essence and mix again. Sieve in the flour and salt and mix thoroughly until combined. Add the chocolate chips and fold through. Spoon this on top of your brownie and Jaffa cakes, smoothing out with a palette knife.
Place the tray into your preheated oven for 1 hour 15 minutes, covering with tin foil after 30 minutes to ensure it doesn't go too brown.
Serve: Take your tray out the oven and slice into 15 pieces and serve whilst still slightly warm.