Preheat oven to 350F (175c, or 160c with a fan) and line a 8x8 or 9x9 baking dish with parchment paper
For the brownie layer: Whisk egg, then add all other wet ingredients, combine well, then add dry ingredients and mix until a batter forms, fold in chocolate chips
Bake for 12 minutes while you make the cookie layer
For the cookie layer: Add wet ingredients to a small bowl, combine well, then add dry ingredients and mix until a batter forms, fold in chocolate chips
Remove brownies from oven and carefully spread cookie layer on top using your hands, top with extra dark chocolate chips
Bake for another 13-15min until tooth pick comes out clean
Let cool for a few minutes and enjoy!