For the biscuit layer, beat together the butter and sugar until light and fluffy.
Add in the egg and vanilla, beat until smooth.
Slowly add the flour, bicarbonate, cornflour, salt and cornflour. Beat until a cookie dough forms.
Fold in the chocolate chips and press the mixture into a lined baking tray. Freeze (or chill) for at least 20 minutes.
After chilling, cover the biscuit layer with oreos (a sawing motion with a knife helps to cut them in half)
Preheat the oven to 180ºC/350ºF/ Gas Mark 4.
For the brownie layer, slowly melt your butter and dark chocolate together.
Whisk together the eggs and sugar into a thick mousse like mixture. Then fold in the chocolate mix.
Add in the flour and cocoa powder.
Carefully add the chocolate chips into the mixture, then pour over the oreo layer.
Bake for 25-30 minutes, or until there is only a slight wobble in the middle when moving the tray.
Cool before cutting up (I put mine in the fridge to help them firm up first).