Over medium heat, combine basil leaves, zest of one lemon and cream in a small sauce pan. Stir several times and allow to heat for about 8-10 minutes. Remove from heat and cover. Chill in refrigerator until cold again. (About 2 hours)
Once cream is chilled, remove basil leaves and toss out. In a medium bowl, beat cream until it makes stiff peaks when you remove the mixer. Set aside.
In another bowl, whisk together condensed milk, vanilla and salt if desired. Slowly fold in the whipped cream into the condensed milk and very gently allow the two to combine.
Fold in half of the other lemon's zest and transfer to a freezer safe container. Sprinkle the rest of the zest on top and freeze for about 4-6 hours.
Before serving, allow ice cream to sit out for about 10 minutes before spooning out, especially if it's been in the freezer for much longer than 6 hours. Enjoy!