- 2 cups whole milk
- 1 cup heavy cream
- 4 large egg yolks
- 2/3 cup sugar
- 3 tablespoons fresh lime juice, preferably Key lime
- 2 teaspoons grated lime zest
- 3/4 cup (about 5) crushed graham crackers (optional)
Meanwhile, in a medium heat-proof bowl, whisk the egg yolks until smooth. Gradually whisk in the sugar until it is well incorporated and the mixture is thick and pale yellow. Temper the egg yolks by very slowly pouring in the hot milk mixture while whisking continuously. Return the custard to the saucepan and place over low heat. Cook, stirring frequently with a wooden spoon, until the custard is thick enough to coat the back of the spoon and it reaches a temperature of 185°F. Do not bring to a boil.
Pour the mixture through a fine-mesh strainer into a clean bowl and let cool to room temperature, stirring every 5 minutes or so. Once completely cooled, cover and refrigerate until very cold, at least 4 hours or overnight.
Gently whisk the lime juice and zest into the base. Pour the mixture into the container of an ice cream machine and churn. (Optional: Just after churning quickly stir in the graham cracker crumbs.) Transfer to an airtight container and freeze for at least 2 hours before serving.