Preheat the oven to 325ºF
Line a baking sheet with a silicone mat or a clean kitchen towel to prevent from sliding and place the ramekins on top.
In a medium pot over medium heat, add cream, vanilla bean, lemon zest and salt; bring the mixture to the point just before it starts simmering. Turn off the heat and let it steep for 5 minutes.
Meanwhile, in a large mixing bowl, combine egg yolks and sugar; whisk until well blended.
Discard the vanilla pod (*see footnote) and slowly pour hot cream into the egg yolk mixture, whisking constantly to temper the eggs. After a few ladles, pour in the warm cream mixture to the tempered eggs. Stir in limoncello liqueur. Strain the mixture through a fine sieve; discard any solids.
Pour custard evenly into the ramekins and carefully transfer the baking dish to the oven.
Pour hot water in the baking dish, 2/3rds of the way up the sides of the ramekins.
Bake until just set with a slight jiggle in the center, about 30 to 35 minutes for shallow ramekins (about 1-inch high) and 35 to 40 minutes for taller ramekins (about 2-inches high).
Carefully remove the ramekins from the water and place them on a wire rack to cool.
When cool, place them on a baking sheet lined with a silicone mat and refrigerate for at least 2 hours and up to 3 days.
Just before serving, evenly sprinkle on 1 tbsp. of sugar per ramekin. Using a torch or the broiler, heat until the sugar melts and caramelizes.