Preheat the oven to 175 C. Line an 8-inch square pan with non-stick baking paper, leaving a little overhang for easy removal. Set aside.
Make the brownie batter. Combine the butter, chocolate and cocoa in a microwave-safe bowl and heat in 10 second increments until smooth. If any lumps of chocolate remain, stir gently to melt them, do not over heat the mixture. You can also use a double boiler for this step. Set the melted mixture aside to cool slightly.
Beat the eggs, sugar and vanilla together with a hand mixer on medium speed until light and frothy, 2 to 3 minutes. Although this step normally ensures a flaky crust, it doesn't matter here because we're covering the brownie batter with cookie dough. However, beating the eggs well also ensures a natural rise and great texture in the brownies.
Pour in the cooled melted chocolate mixture and beat till combined. Sift in the flour and salt and use a spatula to fold it all together until you have a thick, shiny brownie batter. Spread into the prepared baking pan and smoothen the top (see photos below).
Make the cookie dough. You can either rinse out the bowl and beaters that you used for the brownie batter and reuse them here, or use a stand mixer for the cookie dough layer. Beat the butter and sugars in a mixing bowl with a hand mixer on medium speed, until pale and fluffy. Add the egg and vanilla and beat again till combined.
Sift in the flour, baking soda, baking powder and salt, if using. Beat on low speed or fold by hand to combine into a thick, slightly sticky dough. Finally, stir in the chocolate chips.
Now spread the cookie dough evenly on top of the brownie batter.
Bake for 25 to 30 minutes until a toothpick poked in the center comes out with only a few moist crumbs, no wet brownie batter. Remember that over-baking these will lead to the cookie layer drying out and the brownie layer to be more cakey than fudgy.
Allow the brookies to cool for 15 minutes, then lift using the paper overhang and leave to cool completely before slicing into squares. On day one, the center pieces will be softer and slightly under-baked compared to the edge pieces but they naturally firm up by day two.
Store at room temperature for 3 to 4 days or in the fridge for 10 days and in the freezer for longer. Happy eating!