Once it is baked remove the shortbread and place on the counter to cool. While you are waiting begin the caramel sauce. Add the cream, sugar, syrup and butter to the pan and gradually mix together. Suggest using a medium heat and stir continuously. Once the ingredients have mixed smoothly add the salt and vanilla. Taste continuously to sweeten to your own level by adding more sugar. Leave on a low heat until ready, this usually takes between 5 and 10 minutes of slow heating and stirring, about the same length of time it takes for your shortbread to cool. Pour the mix carefully over the shortbread, making sure to cover the whole thing then place in the fridge for approximately an hour and a half to two hours or until the caramel has set sufficiently.