How to Make It
In a heatproof medium bowl, combine both of the chocolates with the butter. Set the bowl over a medium saucepan of simmering water and stir until the chocolate and butter are melted and blended. Remove the bowl and set aside. Pour the water out of the saucepan.
In the saucepan, combine the corn syrup, sugar, salt and 2 cups of water and bring to a boil over high heat.
Reduce the heat to moderate and whisk in the melted chocolate. Cook, stirring occasionally, until the sauce is thick and shiny, 18 to 20 minutes.
Remove from the heat and stir in the vanilla. Use immediately or let cool completely and refrigerate. Rewarm in a microwave before serving.