In a medium saucepan, combine whole milk, sugar, and 1 cup of heavy cream. Heat over medium heat until bubbles form around the edges, stirring occasionally, about 5-7 minutes. Once heated, add fresh mint leaves, cover the saucepan, and let steep for 1-2 hours.

While the mint mixture cools, whisk together the egg yolks in a large bowl until smooth and slightly pale, about 1-2 minutes.

Reheat the mint-infused milk mixture over medium heat until warm but not boiling. Drizzle about one-third of this mixture into the whisked egg yolks while stirring continuously. Return the yolk mixture to the saucepan, cooking and stirring gently until it thickens enough to coat the back of a spoon, about 5-7 minutes.

Strain the custard through a fine mesh sieve into a large bowl to remove the mint leaves. Add the remaining heavy cream and stir well. Cover with plastic wrap and chill in the refrigerator for at least 4 hours, or overnight.

Once chilled, pour the mixture into an ice cream maker and churn according to the manufacturer's instructions, about 20-25 minutes. Transfer to an airtight container and freeze for 2-3 hours.

In a blender, puree the chopped watermelon, sugar, and lemon juice until smooth, around 1-2 minutes. Pour into a shallow dish and place in the freezer.

Every 30 minutes, use a fork to scrape the mixture until fluffy and icy, which should take about 3-4 hours.

Layer spoonfuls of the granita in bowls or glasses, followed by scoops of mint ice cream. Finish with a sprinkle of flaked sea salt and optionally drizzle with sparkling rosé.