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Heat gently (low heat). Stir constantly until the sugar dissolves. Stir constantly; this prevents the sugar from crystallizing.  Stir through. This sauce can be used warm or cold. If you need to store, this sauce will keep for up to 7 days if covered and refrigerated.
Place the butter into a heavy-based saucepan. Add the sugar and cream. Simmer for 8 to 10 minutes over low heat. Remove once the sauce has just thickened. Add the vanilla extract. Serve.