Whisk egg yolks, whole egg, and cornstarch in a medium bowl until smooth.
Put sugar, salt, and 1/2 cup water in a 4-qt. saucepan and bring to a boil over high heat, without stirring but swirling occasionally, and cook until mixture gets thicker, foamy, and lavalike, about 5 minutes. Reduce heat to low and slowly whisk in 3 cups cream and the milk.
Remove from heat and slowly ladle half the cream mixture into egg mixture, whisking constantly. Pour back into saucepan and cook over medium heat, whisking constantly, until mixture is as thick as cream gravy, about 6 minutes. Remove from heat and whisk in butter and whisky.
Pour custard through a fine-mesh strainer into a bowl. Ladle into 8 lowball glasses; cover with plastic wrap. Chill for at least 4 hours and up to several days.
Whisk remaining 1 cup cream in a medium bowl until soft peaks form, then add crème fraîche and whisk until stiff peaks form (this will happen quickly; be careful not to overbeat).
Spoon 2 tbsp. caramel sauce onto each custard, sprinkle with a few flakes of salt, and top with a dollop of whipped cream.
*A flaked salt from England; find at well-stocked grocery stores.