Puree watermelon, sugar, and lemon juice in a food processor until smooth. Transfer to a shallow dish and freeze, scraping every 30 minutes for 3-4 hours until fluffy.

In a medium saucepan, heat whole milk, sugar, and 1 cup of heavy cream over medium heat until it bubbles. Remove from heat, add mint leaves, and steep for 1-2 hours.

Whisk egg yolks until pale. Temper by gradually adding warm milk mixture while whisking, then combine back into the saucepan.

Cook the mixture over medium-low heat, stirring constantly until it thickens, about 5-7 minutes. Strain and stir in remaining cream and a pinch of salt.

Cool the mixture for 30 minutes at room temperature, then refrigerate overnight for best texture.

Churn the cooled base in an ice cream maker for 20-30 minutes until fluffy. Transfer to an airtight container and freeze for at least 4 hours.

Serve by layering granita and ice cream in a dish, optionally adding sparkling rosé and garnishing with mint.