In a medium-sized sauce pan stir together 1/4 cup cold water and granulated sugar until combined.
Set over medium-high heat on the stove top and cook 9 -11 minutes without stirring until mixture is a deep amber color.
Start checking closely at the 8 minute mark.
Meanwhile, warm heavy cream and butter together with sea salt in a small sauce pan over medium heat. (Absolutely do not boil.)
When sugar and water mixture is a deep amber color, slowly whisk in the warm cream and butter mixture.
Whisk over the heat until it's smooth a minute or two, then remove from the heat.
Salted caramel sauce will thicken up as it cools.
When sauce is still warm, spoon over apple cake and cascade down the sides.
(Re-heating the caramel sauce will thin it.)