4 large egg yolks
3/4 cup sugar
1/4 tsp. salt
2 1/4 cups heavy cream
3/4 cup milk
Ice water
3 tbsp. key lime juice
3/4 cup chopped graham crackers

In medium bowl, whisk egg yolks, 1/4 cup sugar and salt. In medium saucepan, whisk cream, milk and remaining 1/2 cup sugar over medium heat until just beginning to simmer, about 5 minutes. Add to egg mixture in a slow stream, whisking constantly, then pour into saucepan. Cook over medium-low heat, stirring constantly, until custard thickens and registers 175 on a thermometer, about 8 minutes.

Strain custard into a bowl set in an ice water bath; stir in key lime juice. Let cool, then cover and refrigerate at least 4 hours.

Process according to ice cream machine instructions. Fold in graham crackers, then transfer to airtight container and freeze until firm, about 4 hours.