Make the Mint Ice Cream
In a saucepan, combine the heavy cream, milk, and fresh mint leaves.
Heat over medium heat until steaming but not boiling.
Remove from the heat and allow the mint to steep for 30 minutes.
Strain out the mint leaves and return the infused cream mixture to the saucepan.
In a separate bowl, whisk together the egg yolks and sugar until pale and creamy.
Slowly whisk the warm cream mixture into the egg mixture.
Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens enough to coat the back of a spoon.
Stir in the vanilla extract and salt.
Chill the custard completely in the refrigerator.
Churn according to your ice cream maker’s instructions.
Transfer to a freezer-safe container and freeze until firm.
Make the Salted Watermelon Granita
Blend the watermelon, lime juice, sugar (if using), and sea salt until smooth.
Pour the mixture into a shallow freezer-safe dish.
Freeze for 1 hour.
Scrape the mixture with a fork to create icy crystals.
Continue freezing and scraping every 30 minutes for 2–3 hours until a fluffy granita forms.
Assemble
Scoop the fresh mint ice cream into serving bowls or glasses.
Top with a generous spoonful of salted watermelon granita.
Garnish with fresh mint leaves if desired and serve immediately.