In small saucepan, combine brown sugar and water.
Bring to boil over medium heat.
Stir frequently until mixture reaches 240°F on a candy thermometer.
Remove from heat, and stir in butter, vinegar, salt, and heavy cream.
Set aside to cool to room temperature. Once cool, stir in rum extract.
Transfer to airtight container and place in refrigerator to cool (about 2-4 hours).