Preheat oven to 350°F. Grease a 9 x 13 baking dish.
Beat softened butter, light brown sugar, 1/4 cup white sugar, and 1/2 tsp vanilla extract together in a large bowl until creamy. (3 - 5 minutes on medium in a stand mixer). Add 1 egg; beat until light and creamy (2 - 3 minutes).
Whisk 1 1/4 cups flour, 1/2 tsp salt and baking soda together in a bowl. Gradually stir flour mixture into butter mixture until dough is combined. Stir chocolate chips into dough. Spread dough in the bottom of the prepared baking dish to cover completely.
Stir 1 cup white sugar, melted butter and 1 tsp vanilla together in a bowl; add 2 eggs and beat well. Mix cocoa into butter mixture until well combined. Stir a 1/2 cup of flour, baking powder and 1/8 tsp salt into cocoa mixture until batter is combined. Pour brownie batter over cookie dough and spread to cover completely.
Bake in the preheated oven until a toothpick inserted into the centre of the brookies comes out clean, 25 - 30 minutes. Cool completely before cutting into bars.