Fill a large bowl with ice water. (you are going to use it to cool the cooked mixture so you have a medium sized bowl that fits inside it, test it out ahead of time)
In a small bowl combine 2 tablespoons (30 ml) of the whole milk with the cornstarch, whisk it until smooth, and set aside.
Measure the sugar into a large, deep saucepan. Place it over medium heat and try not to freak out. Trust the process. Now let the edges start to melt, do not touch it until that starts to happen, no matter how tempting it is. Cook the sugar, stirring frequently and gently, until it starts to brown. You want it to turn a nice amber colour. Once it turns amber, quickly and carefully (caramel bubbles and acts a fool so don't freak out if it starts to bubble up and make some noise) add about ¼ cup (60 ml) of the heavy cream to the saucepan. Remember it's going to bubble and spit, so be careful when doing this step. Stir, the caramel will harden (and you're going to be thinking you ruined but again, trust the process), but it's okay. Add the remaining cream to the saucepan, stirring to combine it.
Now pop the saucepan back over medium heat and add the rest of the whole milk (remember you took 2 tablespoons out of it earlier and reserved the rest), stirring while it combines. Bring the mixture to a boil and cook for about 3 minutes, you want the caramelized sugar to dissolve completely.
Remove from the heat and add the cornstarch mixture you set aside earlier, whisking gradually till combined.
Return to a boil over moderately high heat until the mixture thickens slightly, this should take about a minute.
Pour the mixture into into a medium sized bowl, then whisk in the salt. Place that bowl inside the ice water bath and allow it to cool for about 20 or 25 minutes. Whisk it occasionally as it cools.
Now cover the bowl and pop it in your fridge for about 4 hours, or overnight.