Preheat oven to 180c/350f.
Grease and line an 18cm/7" square (base measurement) cake/baking tin and set aside.
Using a hand held mixer, cream together the butter (75 g) and golden/raw caster sugar (½ cup/100 g) until light and fluffy.
Add eggs (2) and vanilla (1 tsp) and mix to combine, then add cookie butter (1 cup/300 g) and mix until incorporated.
Now folding by hand, add plain/all purpose flour (¾ cup/120 g) and cocoa powder (1 tbsp), and mix to combine.
Fold in chocolate chips, then add mixture to greased and lined baking tin. Top with crushed biscuits (10) and bake for 30 to 35 minutes. Allow to cool before slicing.
Warm the extra cookie butter in the microwave, and drizzle over brownies (optional) to serve.