Preheat the oven to 180°C / 350°F / Gas mark 4.
Line an 8inch/20cm square baking tin with greaseproof paper. Set aside.
Break the chocolate up into a microwave-safe bowl, and add the butter. Melt in the microwave in 30-second blasts, mixing after each, until completely smooth.
In a separate bowl, whisk the eggs and sugar for one minute until pale in colour and frothy.
Pour the melted chocolate and butter mixture into the whisked eggs.
Add the flour and gently fold everything together until fully combined.
Using your hands, break up half of the Biscoff biscuits and stir them into the brownie mixture.
Pour half of the brownie mixture in the baking tin and spread out into an even layer.
Melt 2 heaped tablespoons of the Biscoff spread in the microwave for 10 seconds. Pour on top of the brownie layer.
Pour the remaining brownie mixture on top and spread it out to the edges so that the Biscoff spread is concealed.
Either place the Biscoff biscuits on top or break them up and scatter them over the brownie layer.
Bake for 18-20 minutes* until the top is no longer shiny and small cracks have started to show around the edges. If you give the baking tin a gentle shake, the brownies should wobble underneath.
Leave to cool completely for at least 4 hours or overnight for best results. You can speed this up by putting them in the fridge to set for 2 hours. This will make cutting them easier and make the brownies even more fudgier.