Add the caster sugar and sifted cocoa powder and stir. Heat in the microwave on 30 second bursts (50% power) until the sugar has almost dissolved and the ingredients have combined - stirring each time.
Using a knife or skewer, lightly swirl the Biscoff spread through the brownie batter. Place the roughly chopped Biscoff cookies on top of the brownie. Bake for 30 minutes.