Preheat your oven to 180ºC (160ºC fan) and line a 9x9-inch square baking tin with parchment paper. Melt the dark chocolate and butter together, then set aside to cool slightly while you prepare the rest of the batter. Whisk the eggs and sugar until the mixture becomes thick and mousse-like — it should double in size, lighten in color, and leave a trail that slowly disappears when you lift the whisk. Gently fold the cooled chocolate and butter mixture into the egg and sugar mixture, taking care not to deflate the air you’ve incorporated. Next, add the plain flour and cocoa powder, folding them in carefully. Stir in the chocolate chips and chopped Biscoff biscuits, then pour the batter into the prepared tin. Using two teaspoons, dollop about 15 spoonfuls of Biscoff spread evenly over the top. Bake for 25 to 30 minutes or until the center still has a slight wobble when the pan is gently shaken. Allow the brownies to cool completely in the tin, then chill them in the fridge for a couple of hours to firm up before slicing.