Grease your baking tin all over, then lie a piece of greaseproof paper one way so that it cover the base and sides. Grease this piece of greaseproof paper and then repeat with a piece lying in the opposite way.
Add your butter, condensed milk, milk, double cream and demerara to your heavy based saucepan and heat slowly whilst constantly stirring with your balloon whisk or square based spoon.
Keep heating slowly until you can no longer feel sugar grain crystals on the bottom of the pan and when you hold up a spoonful of the liquid it is also grain free.
Once the sugar has melted, turn up the heat, stirring constantly and rapidly and bring to a rolling boil.
Once the liquid is boiling turn it down so that is simmering and continue to stir rapidly, making sure you continually scrape the bottom of the pan to ensure the mixture does not burn.
If you see flecks of brown rising this means it is sticking a little, take it immediately off the heat and continue to rapidly stir for a couple of minutes, then return to the heat but repeat if necessary. These brown flecks will disappear when you beat the mixture in the next stage.
It will take 10-15 minutes for the mixture to reach the required temperature. Keep testing it with your sugar thermometer, once it reaches 115°C it is ready. If you don't have a thermometer take a little mixture onto a teaspoon and drop it into your iced water it should form a ball when but do BE CAREFUL IT IS EXTREMELY HOT!
Once the fudge is ready keep on rapidly stirring for another 2-3 minutes otherwise it will still catch and burn on the bottom.
Then set aside to cool until still just warm.
Add the mixture to a bowl of your stand mixer.
Turn the speed up to full and beat until the mixture becomes much thicker, ribbon like in texture, paler and glossy. See pictures in this post so you know what to look for.
On a low speed beat in your Biscoff spread and vanilla bean paste (or extract) until incorporated.
Once the Biscoff fudge has reached this consistency pour it immediately into your prepared tin.
Take your Biscoff biscuits and crush them until they are sand like in consistency (either in a pestle and mortar or put them into a sandwich bag and bash them with a rolling pin).
Sprinkle over the top of the fudge, press in with the back of a spoon to set them.
Leave the Biscoff fudge to set for about 3 hours, then slice it up.