Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" Square Pan.
Start by melting the dark chocolate: break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn. Set aside to cool a little whilst doing the rest:
Combine the melted butter, granulated sugar and light brown sugar in a large bowl/bowl of a stand mixer – on a medium speed for 3-5 minutes.
Add in the eggs and vanilla and mix again. Then, checking first that the chocolate isn't so hot as to cook the eggs, add it in and keep mixing.
Fold in the flour and cocoa powder.
Remove approx 9 cookies from the packet and set aside for later. Roughly chop the remaining cookies. Add the flour, cocoa powder, chopped chocolate and chopped cookies into the batter and fold in gently.
Pour batter into the pan. Add the cookies saved from earlier on top - just lightly place them on the surface of the batter. Bake 30 minutes (should no longer be wobbly), then remove and let cool on a wire rack before slicing!