Recipe adopted from Giada De Laurentiis.
14 ounces sweetened condensed milk
2 teaspoons ground cinnamon
1 teaspoon pure vanilla extract
2 1/2 cups semisweet chocolate chips
3 tablespoons unsalted butter, cubed and brought to room temperature
Kosher salt (optional)
Butter the bottom and sides of an 8"x8" pan and line with a sheet of parchment paper (to allow for easy removal of fudge once cooled).
In a saucepan, heat a couple cups of water until barely simmering. While the water is heating, add the milk, cinnamon, and vanilla to a medium (heatproof) bowl and stir. Add the chocolate and butter, and move the bowl onto the saucepan to create a double-boiler. Over a gentle simmer, continue to stir ingredients with a rubber spatula until everything has melted. It is important to make sure that the water doesn't boil and that the water doesn't touch the bottom of your pan, as these will both cause your chocolate to overheat and turn grainy.
Once a smooth consistency is achieved, transfer mixture to the 8"x8" pan, lightly sprinkle with Kosher salt (if desired), and cool in the refrigerator for several hours before cutting and serving.