Combine the sugar, xanthan gum (if using), cornstarch and salt in a medium pot. Whisk to combine well. Pour in a bit of the milk and stir together to combine until smooth. Pour in the remaining milk and the golden syrup and whisk together.

Place the pot over a medium-low flame on the stove and heat, stirring often with a whisk, until the mixture is hot and has thickened slightly. Stir in the pistachio paste until smooth and remove from the heat. Stir in the double cream.

Let the mixture cool to room temperature. Either churn it now (see next step) or chill the mixture overnight ('ageing' the ice cream base overnight like this improves the texture of the ice cream).

When ready to churn, pour the ice cream base into your ice cream maker and churn according to the manufacturer's instructions. You want to stop it churning once it reaches the texture of soft-serve ice cream. Scoop all the churned ice cream into a plastic tub, seal with a lid and chill for at least 2 hours before scooping and serving.