Heat oven to 180°C (160°C fan forced). Meanwhile, grease a 20cm square tin with butter and line with 2 long strips of folded baking paper.
Place the butter and chocolate in a medium-sized microwave bowl and microwave at 30 second intervals until melted. Leave the melted chocolate mixture to cool to room temperature.
Break the eggs into a large bowl and add the caster sugar. With an electric mixer on maximum speed, whisk the eggs and sugar for 5-8 mins or until thick and creamy and doubled in size.
Pour the cooled chocolate mixture over the egg mixture and gently fold together.
Sieve the flour & cocoa into the wet ingredients. Fold the mixture gently with a spatula until just combined (do not over-mix).
Pour the mixture into the prepared tin and carefully level the mixture with the spatula. Chop up your preferred frozen cookies into quarters and scatter over the top of the brownie mixture.
Place in the middle of the oven and bake for 40-45 minutes.
Remove tray from the oven, cool on a rack, then slice.