Add the eggnog to a medium bowl and sprinkle the gelatin over the top. Whisk, then allow the mixture to sit for 18-20 minutes.  

Meanwhile, warm the milk, sugar, and salt in a saucepan over medium heat. Once steaming and barely beginning to simmer, add the eggnog and gelatin mixture to the pan to make the panna cotta base. Return to a simmer and cook for 3-4 minutes, whisking constantly, until the gelatin is entirely dissolved. Remove from the heat.

Place the sour cream in a large bowl. Strain the panna cotta base through a fine mesh sieve over the sour cream. Whisk to combine and allow to cool slightly for about 5 minutes.

Lightly butter a 1.5-quart ramekin. Pour the panna cotta base into the ramekin, then wrap tightly with plastic wrap and transfer to the fridge. Chill for at least 8 hours, or until completely set. 

To serve, loosen the sides of the panna cotta with a knife and set the bottom of the ramekin in hot water for about 1 minute. Invert onto a serving platter or cake stand, then spoon caramel sauce over the top. Finish with a sprinkle of flakey salt, if desired, and enjoy immediately.