In a small saucepan over medium low heat, add the butter, brown sugar, and sweetened condensed milk. Stir. Once everything has melted, add the cinnamon and vanilla. Stir to combine.
Remove from heat and serve. The dip will thicken as it cools. *
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Notes
*I prefer eating this apple dip warm. It's amazing when it's warm. Store covered in the fridge for up to 2 weeks.