Butter the bottom and sides of an 8 by 8-inch baking pan. Line the pan with a sheet of parchment paper, about 14-inches long and 7-inches wide, allowing the excess to overhang the sides. Set aside
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 the prepared pan and smooth the top. Sprinkle with salt, if desired. Refrigerate for at least 2 hours until firm.
Run a warm knife around the edge of the pan to loosen the fudge. Remove the fudge to a cutting board. Peel off the parchment paper and cut the fudge into 1-inch pieces. Store refrigerated in an airtight container or freeze.
The fudge can also be made using 1 pound (about 2 1/2 cups) of semisweet chocolate chips.