Preheat oven to 160C (if you're not sitting at a campfire, that is!). Line a baking tray with baking paper.
Spread 4 of the biscuits with a little bit of Biscoff spread and place them on the baking tray. Break the chocolate into four smaller squares (as you see in the photos above) and arrange them close together on top of the spread. Cut the marshmallows in half and place two, sticky side up, on top of the chocolate of each biscuit. Lie the remaining 4 biscuits on top of the marshmallows and push down gently.
Carefully put the tray in the oven so the s'mores won't topple over, then bake for 4-6 minutes. Keep your eye on them constantly as when the marshmallows start melting, they'll melt really quickly. Take them out when you see the chocolate has melted and the marshmallows have melted only a little bit. They'll continue to melt as they sit.
They'll be messy to eat, but so delicious! Enjoy!
Biscoff is also known as Speculoos, or Speculaas, or sometimes just by the company name, Lotus.