Add all of the caramel sauce ingredients to your heavy-bottomed saucepan and heat on a low setting until fully melted and the all of sugar has completely dissolved, mixing gently with a balloon whisk at all time. You must be sure that the sugar has dissolved to avoid the caramel taking on a grainy texture. To check this tip the pan to the side and look for any grains remaining on the base.
Whisking rapidly, turn the heat up so that the mixture begins to boil, as soon as it hits boiling point turn the heat back down to a medium heat and keep at a good simmer. Whatever you do don't stop that rapid whisking, the sauce will very easily catch on the bottom of the pan otherwise.
Continue to whisk the mixture rapidly whilst it is simmering for about 6-7 minutes until the caramel has turned light brown, it is gloopy and resembles a toffee sauce.
Continue to whisk away from the heat for a minute or so until it stops bubbling and then pour over your cooled biscuit base and level out.
Place in your fridge to set for an hour.