Preheat your oven to 180ºc/160ºc fan, and line a 9x9" square tin with parchment paper.
Melt the dark chocolate with the butter and leave to cools lightly whilst you make the rest of the brownies
Whisk together the eggs and sugar until really thick and mousse like - it will double in volume, be a lot paler, and leave a trail for a couple of seconds before disappearing when done
Fold the chocolate/butter mix into the eggs/sugar mix carefully - don't deflate the eggs by being careless!
Add in the plain flour and cocoa powder, and fold that in as well.
Add in the chocolate chips and Biscoff biscuits, fold through, and pour into the tin.
Using two teaspoons, spoon dollops of Biscoff spread onto the top of the brownies - I do about 15 or so dollops!
Bake the brownies in the oven for 25-30+ minutes, until there is a small wobble in the middle of the tray.
Leave the brownies to cool fully in the tin and then set them in the fridge for a couple of hours to firm up
Notes
These will last for 4-5+ days at room temperature or in the fridge
You can freeze these brownies for 3+ months
Smooth or crunchy biscoff spread works well
You can swap the spread for other spreads such as Nutella, or even peanut butter