Chop the basil leaves finely to release their fragrant oils and set aside.
In a blender or food processor, combine the heavy cream, sweetened condensed milk, lemon zest, and chopped basil.
Blend everything on high until the mixture is smooth, the basil is finely broken down, and the lemon zest is evenly distributed, about 30 seconds.
Pour the mixture into a freezer-safe container, spreading it out evenly with a spatula.
Cover the container with a lid or plastic wrap, ensuring it’s sealed tightly to prevent ice crystals.
Freeze for at least 4 hours or overnight, until the mixture is firm and scoopable.
Once frozen, scoop the ice cream into bowls or cones and enjoy the bright, herbaceous flavor with a creamy texture.