Line an 8-inch square pan with foil, allowing excess to hang over the sides. In a small bowl, stir together the sugar and cinnamon.
Combine the white chocolate chips, sweetened condensed milk, and salt in a medium saucepan over low heat. Stir frequently until melted and smooth (the mixture will be on the stiff side), then remove from the heat and stir in the vanilla extract.
Transfer the mixture to the prepared pan and smooth into an even layer.Sprinkle the cinnamon-sugar mixture evenly over the fudge. Refrigerate for at least two hours.
Using the aluminum foil, lift the fudge from the pan, then cut into squares. The fudge can be stored in an airtight container at room temperature for up to two weeks. Alternately, you can store the fudge in the refrigerator for up to 1 month, or in the freeze for up to 2 months.
Notes
You can use a 9-inch square pan, but your fudge will be a little shorter.