Add your caster sugar, golden syrup, vanilla extract and soy cream to a saucepan and place over a low heat. Stir until fully combined and then stir in the butter and bring to a simmer on a medium heat.
Simmer for around 15 minutes, continuing to stir constantly so it doesn't stick or burn. The caramel will start to thicken up but will still be slightly runny. You wil be able to tell it's ready by dropping a small amount into a bowl of cold water, it should form a soft caramel ball when pressed together.
Pour the caramel on top of the shortbread biscuit and place in the fridge to set for atleast an hour.