Line a 13×9-inch baking pan or casserole dish with wax paper and lightly spritz with cooking spray.
Place sweetened condensed milk, white chocolate chips, 3 tablespoons butter, rum, vanilla, and 1/2 teaspoon of cinnamon in a medium saucepan over low heat, stir to combine. Alternatively, you can use a double boiler.
Melt chips, stirring frequently, until smooth, about 5-6 minutes.
Remove from the heat and pour into the prepared baking dish. Spread the fudge mixture evenly along the bottom of the pan and set aside to cool at room temperature.
Once the fudge has cooled to room temperature, melt the remaining 3 tablespoons of butter in the microwave, for 20-30 seconds, and brush over the surface of the fudge.
Combine the remaining 1/2 teaspoon of cinnamon with the sanding sugar and sprinkle over the fudge, cover the top as thoroughly as possible.
Cover the fudge and place in the refrigerator until firm, about 3 hours.
Cut fudge into squares before serving.