In a saucepan, combine milk, cream, half the sugar (about ⅓ cup), and lemon zest. Heat over medium until steaming but not boiling.
1 cups whole milk, 2 cups heavy cream, ¾ cup granulated sugar, 1½ tablespoon lemon zest
In a bowl, whisk egg yolks, remaining sugar, and salt until pale and thick.
4 large egg yolks, Pinch of salt
Slowly stream the warm milk/cream into the yolks while whisking. Return to the saucepan.
Stir constantly over medium heat until it thickens and coats the back of a spoon (170–175°F). Remove from heat.
Strain through a fine mesh sieve. Stir in lemon juice and vanilla.
½ teaspoon vanilla extract, ½ cup fresh lemon juice
Cover and chill the base for at least 4 hours or overnight until very cold.
Churn in your ice cream maker until soft-serve consistency (about 20–25 minutes).
Layer half the ice cream into a container, dollop in half the lemon curd (optional), and swirl gently. Repeat with remaining ice cream + curd. If you're not using the curd, just fill your container with the ice cream after churning it.
½ cup lemon curd
Cover and freeze 4+ hours or until firm.