Preheat your oven to 160℃ (140℃ for fan ovens) then line an 8x8-inch tin with baking paper. Set aside for now.
Melt the dark chocolate and the butter together over low heat on the stove then set it aside to cool for around 10 minutes.
To a large bowl, add the sugar, eggs and vanilla extract. Use an electric whisk to whisk it all together until pale and silky.
Stir the melted chocolate mixture into the egg and sugar mixture. Then add the flour and the cocoa powder and mix gently until well combined.
Set aside 9 of the bourbon biscuits and crush the rest into chunks. Fold the chunks into the brownie batter along with the chocolate chips.
Place the remaining 9 biscuits onto a baking tray and pop them into your preheated oven for a few minutes until the filling has melted. This will enable you to just slide them apart, creating 18 biscuit halves.
Spoon the batter into your lined tin and spread it out evenly. Top with the bourbon biscuit halves, placing them in 3 rows of 6.
Bake the brownies for around 35-40 minutes and leave them in the tin to cool. Then transfer them to the fridge overnight (or for at least 6 hours), until solid.
Once set, cut them into slices using the biscuits as your guide and enjoy!