Measure all the ingredients into a medium-large saucepan, preferable stainless-steel if you have one.
Place the saucepan over a low heat and stir with a wooden spoon or spatula until the ingredients have melted into a smooth mixture.
Turn the heat up to medium so that the mixture starts to bubble up. Once bubbling set your timer for 5 minutes and keep stirring continuously until the caramel thickens and turns golden in colour.
Take the saucepan off the heat and stir the caramel for 30 seconds to knock out any air pockets. The caramel will darken in colour and continue to thicken as it cools down.
Pour the caramel over the shortbread and spread out into an even layer.
Leave to cool at room temperature for 30 minutes. Then place in the fridge for 1 hour to set.