Preheat the oven to 180C (350F). Line a slice tray approximately 9×13 baking tin with baking paper.
Sift the flour, baking powder and cocoa together, then add both sugars and mix well. Make a well in the centre and add the beaten eggs, melted butter and vanilla. Gently stir until it is all incorporated (but don’t overmix).
Pour half of the mixture into the prepared pan and spread to all the edges. Lay the 12 cookies out side by side on top of the first layer of brownie mix. Pour the rest of the brownie mix on top of the cookies and spread to the sides and corners.
Bake for around 28 minutes until firm around the edges but the middle should still be a little soft (but not wobbly).