Have insulated ice cream container(s) with lid(s) ready, or line a 9 x 5-inch (23 x 13 cm) loaf tin with parchment paper and set aside.
Stir the condensed milk, vodka (if using), vanilla, and salt together in a small to medium bowl until well combined, then set aside.
Add the whipping cream to a large mixing bowl or the bowl of a stand mixer. Using a hand or stand mixer on medium-high speed, whip the cream to medium-stiff peaks, about 3 to 4 minutes.
Scrape the condensed milk mixture into the bowl of whipped cream. Continue whipping until the mixture is light, fluffy, and fully combined, stopping to scrape down the sides of the bowl if needed.
Dollop one-third of the ice cream base into the bottom of the prepared container. Spread the surface evenly, then dollop one-third of the lemon curd over top and swirl it gently with the tip of a butter knife. Repeat twice more with the remaining ice cream base and lemon curd to make a total of three layers.
Cover the container and freeze for 6 to 8 hours, or until firm enough to scoop.
Notes
Whip the cream to medium-stiff peaks. It should hold its shape but still look soft on the tip of the beater.
If you’re not sure whether the condensed milk mixture is thoroughly combined, stop and use a spatula to gently fold it through.
Use a light hand when swirling the lemon curd so the swirls stay defined.
Before scooping, let the container sit at room temperature for 5 to 10 minutes to soften slightly.
Storage
Store the ice cream in an airtight, insulated container or covered loaf tin in the freezer for up to one month.