Melt Butter: In a small saucepan, melt the butter over medium heat.
Add Sugar and Cream: Add the brown sugar and heavy cream to the melted butter. Stir until combined.
Simmer: Bring the mixture to a gentle boil, stirring constantly. Lower the heat and let it simmer for 4 minutes, stirring occasionally, until slightly thickened.
Add Rum and Salt: Remove from heat and stir in the rum and salt carefully (it will bubble and sputter step away to avoid burns).
Cool Slightly: Allow the sauce to cool 5-10 minutes before serving. It will thicken as it cools.
Note: if caramel gets too thick add 2 T cream to thin and warm slightly.