to make caramel: melt sugar, then quickly add salt + rum + cream.

Not butter. Not water.

After it melts, there isn't time to use a candy thermometer. 

I used a small All-Clad sauce pan. Make sure the sugar is under 1/4 the height of the pan, because there will be a dramatic bubbling up when you add rum and then cream to the melted sugar.

To make caramel sauce:

• Melt sugar (~ 1.5 cup) without stirring.* 
• Turn off heat
• Sprinkle in kosher salt (1 Tablespoon?)
• Stir in rum (3 Tablespoons? Really, as much as you might pour in cautious speed ready to stir it in immediately.)
• Stir in light cream (~ 1/2 cup?? I needed to use up the pint.)
• Once you can stir smooth, pour it into a jar.

*Today (5th of January) I learned from Milk Street Radio that stirring during the melting step leads to crystalization. But do have something like a long wooden fork that is good for quickly incorporating ingredients ready for the stirring steps, which are dramatically bubbly and best completed ASAP.