• Line a 9x9 inch tin with greaseproof paper. Food52 has a great how to here.

Let's make the cookie dough layer first

• Cream the soft butter and sugars.
• Add the egg and fully incorporate.
• Next, add your dry ingredients; baking powder, bicarb of soda, salt and flour. And combine.
• Lastly, fold through your chocolate chunks.
• Press the cookie dough into your prepared tin in a nice, even layer, all the way out to the corners.
• Put your tray in the fridge. Preferably overnight, but at least four hours. Means a chewier cookie layer.
• Before we make the brownie layer, preheat your oven to 180°C/160°C fan.

Brownie layer

• Melt the butter and chocolate together. I do this in short blasts in the microwave with lots of stirring. Allow to cool slightly.
• In another bowl, beat the sugar and eggs.
• Add your cooled chocolate mixture to your eggy mixture and combine carefully.
• Finally, fold through your flour, cocoa, salt and chopped chocolate.

Let's put it all together

• Get your chilled cookie dough from the fridge and arrange the Oreos across the whole surface as tightly as you can.
• Pour over your brownie mixture. Make sure to spread it carefully all over without disturbing the Oreos.
• We're ready to bake! Put it in your preheated oven for 20-25 minutes. You want the surface to be set, but still be gooey inside.
• Let them cool completely. I know it's hard. I even fridge mine overnight. If you cut them while they're warm, they'll fall apart. Will be a delicious mess all the same.