Preheat the oven to 160°C. Grease and line a medium size (about 20cm by 25 cm) baking or ceramic tray with baking parchment .
Then, prepare the cookie dough. In a bowl, beat the butter and sugar until pale and fluffy. Add the egg and the self-raising flour. Once combined, fold in the chocolate chips.
Flatten between 2 sheets of parchment paper to the size of the baking tray. Place in the freezer or fridge to set while getting the brownie mixture ready
While the cookie dough is setting, prepare the brownie mixture.
Place a saucepan of simmering water, over low heat. Place a bowl and add the chocolate pieces. Melt the chocolate and keep aside until needed.
In a bowl, beat the butter with the sugar until pale and fluffy. Add the eggs and flour and cocoa powder and again combine until smooth.
Stir in the melted chocolate into the cake mixture. Pour into the lined baking tray.
Carefully place over the brownie mixture the cooled cookie dough.
Transfer into the oven and bake for 35-45 minutes until cooked through.
Remove from the oven and allow to cool completely in the tin before portioning.
Store in a tight container and enjoy within a few days.