Put the chopped chocolate into a saucepan with the golden syrup, milk and cream.
Heat very gently, stirring all the time until the chocolate has completely melted. Take care not to over heat the mixture.
Remove the pan from the heat and gradually whisk in the butter. Add a couple of drops of vanilla extract, to taste.
Pour into a jug. Serve warm or cold drizzled over ice cream (the sauce can be made-ahead and kept covered and chilled in the fridge for up to 5 days).
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