Preheat the oven to 325ºF and line a baking sheet with a silicone mat or a clean kitchen towel so they don’t slide. Set 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 vanilla pod and slowly pour hot cream into egg yolk mixture, whisking constantly to temper the eggs. Stir in limoncello. Strain mixture through a fine sieve; discard. Pour custard evenly into ramekins and carefully transfer the baking dish to the oven. Pour hot water 2/3 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 ramekins from water and cool on a wire rack. When cool, place on a baking sheet lined with silicone mat and refrigerate for at least 2 hours and up to 3 days.
Just before serving or up to 2 hours before serving, sprinkle sugar (1 tablespoons per shallow ramekin) evenly over the surface. Using a torch or broiler, heat until the sugar melts and caramelizes. Makes 6 ramekins.