In a medium saucepan, melt butter on low heat.

Add brown sugar, corn syrup, and milk, stirring to combine. Increase heat to medium (or maybe a little below medium).

Stir constantly until mixture comes to a boil. Make sure to use a heat resistant spoon.

Remove from heat and whisk in the vanilla and (optional) cinnamon.

Serve warm or cold over apple slices, ice cream or whatever you fancy!