Combine the sugar and cocoa powder in the top of a double boiler set over briskly simmering water, and whisk to combine.  Add the heavy cream and corn syrup, and whisk until smooth.  Cook, whisking constantly, until the mixture is hot, thick, and creamy, 3 to 5 minutes.  Add the chocolate, butter, vanilla, and salt, and stir to combine.  Cook, stirring occasionally, until the chocolate and butter are melted and the sauce is smooth, 2 to 3 minutes.  Keep warm until ready to serve. (The sauce can be prepared up to several weeks in advance and reheated gently in a double boiler before serving.) Store it in a nonreactive airtight container, refrigerated.