Put the hot coffee and some Chocolate-Toffee Syrup in a large mug and mix until combined. Stir in the hot milk, top with some whipped cream and drizzle with more syrup, if desired.
Cook the sugar in a medium, heavy saucepan over medium heat, undisturbed, until it begins to melt, then continue to cook, stirring slowly with a fork, until melted and pale golden. Continue to cook, swirling the pan, until the sugar is a deep golden caramel.
Remove from the heat and carefully add the warm cream down side of pan (the mixture will bubble and steam vigorously), return the caramel to the heat, add the chocolate and salt and stir until smooth. Remove the mixture from the heat and stir in the vanilla.