At LEAST 24-48 Hours Before, Freeze Ice Cream Maker. If needed, make sure to freeze the bowl or insert for your ice cream maker in advance. Otherwise, the ice cream will not churn properly.
Combine Sugar & Lemon Zest in a medium saucepan and rub the zest into the sugar with clean fingers until fragrant and yellow. This helps infuse the sugar with lemon essential oils & intensifies the lemon flavor in this recipe.
Add Cream, Milk, Corn Syrup, and Salt to the lemon sugar mixture and stir to combine.
Steep. Bring this mixture almost to a simmer over medium-high heat, until the mixture is steaming and sounds like it's about to bubble. (The sugar should be fully dissolved.) Remove the pan from the heat, stir in vanilla extract, and let it steep at least 30 minutes as it cools. Transfer to an airtight container or liquid measuring cup and cover.
Chill Ice Cream Mixture at least 4 hours, or until very cold. The flavor will continue to develop over the 4 hours. (Don't rush here! If you don't fully chill this mixture, the ice cream will not churn properly!)
STRAIN the ice cream mixture through a fine mesh sieve to remove the lemon zest (otherwise, it can get caught in the churning apparatus of the ice cream maker)
Churn Ice Cream according to your ice cream maker's instructions. For me, that means placing the chilled insert into the ice cream maker, turning it on and pouring the cold ice cream mixture into the chilled insert. Cover and let it churn 20-30 minutes, or until the texture of rich soft-serve ice cream.
Layer Ice Cream. Transfer half of the ice cream mixture into a freezer-safe container. Dollop 1/2 of lemon curd across the surface and cover with the other half of the ice cream mixture. Dollop with the remaining lemon curd and use a butter knife to gently swirl the mixture in a few figure-8 motions. Try not to over swirl!
Freeze. Cover the container well and freeze ice cream 4-6 hours, or until firm enough to scoop.
Serve & Store. Scoop and enjoy! Leftover lemon ice cream will keep up to 1 week in the freezer, though it will continue to harden with time.