Special equipment: an ice cream maker
Place the basil leaves in a medium bowl. Pour 2 cups boiling water over the leaves and leave to steep for at least 20 minutes. Remove the basil leaves, squeezing them over the water to release all of the juices.
Prepare a large bowl of ice water and set aside. Pour the basil water into a small saucepan, add the sugar and bring to a boil. When the sugar has dissolved, remove from the heat and stir in the lemon juice and zest, corn syrup, limoncello and salt. Transfer to a medium bowl and set in the bowl of ice water. Stir until cold.
Pour the lemon mixture into an ice cream machine and churn according to the manufacturer's directions. Transfer to a chilled, airtight container and store in the freezer until firm, 3 to 4 hours.
Scoop the sorbet into serving dishes and garnish with lemon zest and fresh baby basil leaves.
The tartness will vary depending on the lemons you use. Taste the mixed base once it is chilled. If it is too sour, add corn syrup a tablespoon at a time to meet your liking.