In a medium glass or stainless steel bowl, combine the condensed milk, cinnamon, and vanilla. Stir in the chocolate chips and butter.
Put the bowl on a saucepan of barely simmering water
and mix until the chocolate chips have melted and the mixture is smooth, about 6 to 8 minutes (mixture will be thick).
Using a spatula, scrape the mixture into a greased 8X8 pan which has been greased and lined with parchment paper.Make sure to cover it well since you will be lifting it out of the pan using the parchment paper.
and smooth the top. Sprinkle with salt, if using .(I did not do this for this recipe since I did not have the kosher salt on hand - no loss!)
Refrigerate for at least 2 hours until firm. It took me little more than that to set - I think about 3 hours.
You can now cut the fudge into small pieces - either right in the pan - or better remove the parchment from the pan and then cut into slices.
If you greased the paper well, you will see that you can easily peel the parchment from the fudge.