Place the chocolate and butter in a medium saucepan over low heat. Using a double boiler also works well.
Cook and stir continually until chocolate and butter are melted and smooth.
Add sugar and whipping cream and whisk.
Cook on medium heat until the cream barely starts to boil and mixture is smooth.
Continue whisking and cook for 3-4 minutes, until mixture is heated through and smooth.
Remove from heat, add vanilla and stir.
The hot fudge will start to thicken as it cools.
Reheat in the microwave for thirty seconds at a time, then stirring, until desired temperature and consistency is reached.
Notes
Store any leftovers in the refrigerator.I find heavy whipped cream adds richness to this recipe, but whipping cream can also be used with great results.