In a small saucepan combine the butter and brown sugar over medium high heat. Stir until the mixture comes to a gentle boil.

Reduce heat to medium low and add in the sweetened condensed milk and heavy cream. Mix well.

Bring the mixture to a low simmer, stirring constantly, then remove from heat.

Stir in the salt.

Pour into a half pin mason jar and cool before storing in the refrigerator.