Combine the 2 sugars, molasses, cream and coarsely chopped chocolate in a saucepan. Cook over a moderate heat, stirring until sugar and chocolate have melted. Continue cooking, WITHOUT STIRRING, until mixture reaches 238 degrees or until a few drops tested in cold water form a soft ball. Remove from heat, stir in butter and vanilla, cool slightly, then beat until fudge begins to harden. Pour onto a buttered platter and cut into squares before the fudge is completely hard. Makes about 1 1/4 lbs.