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 volume, lighten in color, and when you lift the whisk, the mixture will leave a trail that slowly disappears. Gently fold the cooled chocolate and butter mixture into the egg and sugar mixture, being careful not to deflate the air you’ve whipped in. Next, sift in the plain flour and cocoa powder and fold them in gently as well. Stir through the chocolate chips and chopped Biscoff biscuits, then pour the batter into the prepared tin. Using two teaspoons, drop dollops of Biscoff spread evenly across the top — about 15 dollops works well. Bake in the oven for 25–30 minutes or until the center has a slight wobble when you gently shake the pan. 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.