In a microwave-safe bowl, add white chocolate, butter, and sweetened condensed milk. Heat in the microwave for 30 seconds on 50% power, then stir. Continue to heat for 30 second intervals, mixing in between, until chocolate has completely melted.
Quickly add rum, vanilla extract, and salt to fudge mixture, then stir thoroughly until white chocolate has absorbed the rum and extract.
Pour fudge in the prepared baking dish and smooth out the top in an even layer. If desired, dust the top of the fudge with ground cinnamon.
Refrigerate fudge for at least 3 hours or until set.
Lift fudge out of baking dish by gripping excess paper/foil along the sides, then transfer fudge to a work area. Peel back paper/foil from edges of fudge, then cut fudge into 1 inch squares or cut with festive cookie cutters.