
Measure all the ingredients and place them in separate bowls. Make sure the butter is softened. Please note, that the heavy cream should be warm when added to the sugar, so make sure it is heated prior to this so that it will be warm when the time comes. It can be heated in the microwave.
Place the sugar, water and corn syrup (if using) in a large saucepan. Swirl to moisten all of the sugar.
Cook the sugar over medium-high heat until the water comes to a boil and the sugar dissolves. Remember to swirl the pot to evenly dissolve the sugar. Do not stir, as that will cause the sugar to re-seize.
Place the lid on the saucepan and let the sugar syrup cook with the lid closed for about 2 minutes. This is to allow any sugar crystals stuck on the side of the pan to be washed down back into the syrup and dissolve to prevent recrystallization.
Remove the lid and continue to cook the sugar until it starts to turn light yellow in color. Swirl the pot occasionally to make sure the sugar cooks and colors evenly.
If the whipped cream is not warm, heat the heavy whipping cream in the microwave until warm now (very quickly), as mentioned above in step 1.
When the sugar starts to color, it will continue to deepen in color very quickly. So do not leave the sugar unattended at this stage. Swirl the pot frequently to make sure the sugar cooks and colors evenly without any burn spots.
As soon as the sugar caramelizes to a really dark amber color, remove the pot from the heat and add the warm cream. Stand back as you pour the cream slowly (do not dump it all in at once!) because the hot caramel sauce will bubble up a lot.
Whisk the cream in, and follow with the softened, unsalted butter and salt.
Return the pot to the stove (medium heat) and continue to whisk, in order to dissolve the butter and mix it smoothly into the caramel. Bring the sauce back to a boil while whisking.
Let the sauce gently boil for about 1 - 2 minutes while whisking.
Remove from the heat. Add vanilla and stir to mix in. To taste the caramel, place a small amount of caramel on a saucer / small bowl and let it cool down for a few seconds. Taste and add more salt if needed. Stir to dissolve.
When the sauce has cooled slightly, transfer to glass jars.
Allow the caramel to cool to a slightly warm temperature. Serve while still warm.