Place the chocolate and butter in a medium size saucepan over low heat on stove, or if you are worried about heat control, place in a double boiler. Simmer 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. Cook for about 3-4 minutes until mixture is heated through and smooth. Remove from heat. Add vanilla.