Pre-heat the oven to 190°C / fan 170°C / gas mark 5 then grease and line your tin.
To make the cookie mix, beat together the butter and sugar until light and creamy. Add your egg yolk and vanilla extract. Stir in the flour, salt and baking powder. Then using your hands, bring together the mixture to form a dough. Add ¾ of the chocolate chips (75g). Set aside the cookie dough for later.
In a separate bowl, make up the Dr. Oetker Chocolate Fudge Brownie according to the packet instructions.
Pour the brownie mixture into the prepared tin and spread evenly using the back of a spoon.
Using the palm of your hands, roll the cookie dough mixture into 16 small grape sized balls and place them into the brownie mix, ensuring an even spread. Scatter the remaining chocolate chips over the top.
Bake in the middle of the oven for 24-26 minutes or until the brownies have a papery top and the sides have started to come away from the tin. The middle of the brownie should still be soft and gooey. Remove from the oven and leave to cool in the tin for 10-15mins before cutting into squares.
Enjoy your brookies - the delicious combination of brownies and cookies all in one gooey bake!