Combine the sugar, whipping cream, butter, vanilla, and salt in a medium saucepan.
Bring to a simmer and cook, stirring constantly, until the sauce begins to thicken and coats the back of a spoon, about 5 minutes. Remove from the heat. The sauce will thicken more as it cools.
Make the hot chocolate
In a small saucepan over low heat, warm the milk until hot but not boiling, stirring occasionally. You should see steam coming off the top and small bubbles forming around the edges of the pot.
Remove from the heat and immediately whisk in the chopped chocolate until melted and smooth.
Pour 2 tablespoons of caramel sauce into the bottom of each mug. Pour the hot chocolate over the caramel and stir well to combine.
Serve immediately, topped with whipped topping, if using, and an extra drizzle of salted caramel.
Notes
Prepare the caramel sauce up to 1 week before using. Store it in the fridge in a sealed container.
Extra caramel sauce is terrific for drizzling over ice cream, or use it to make caramel apple dip.
Chop your chocolate very finely so that it melts quickly and easily.