In a heavy saucepan, cook the sugar and cream over medium heat until the mixture begins to simmer. Cover, reduce heat to low, and simmer for 2 minutes. Uncover and cook, stirring occasionally, until the temperature on the candy thermometer reaches 238℉. (After uncovering the pot, you may need to raise the heat to medium-low towards medium, depending on your stove, to maintain a simmer.)

Remove from the heat and drop the butter on top, but do not stir. Let sit for about an hour or until mixture is approximately 110℉. Add vanilla and rum. Beat until frosting reaches a spreadable consistency. (If the frosting does not thicken, chill briefly and beat again. If you lost track of time - as I have many times - and the frosting has chilled too much, warm slightly and beat again.)

Store in a container until ready to use, placing plastic wrap directly against surface. Keep at room temperature for up to 1 week or in the refrigerator for up to 3 weeks. If refrigerated, bring to room temperature and stir prior to frosting your cake.