In a medium saucepan, combine the heavy cream, whole milk, and chopped fresh mint leaves. Heat over medium heat until the mixture just begins to simmer around the edges. Remove from heat, cover, and let steep for 30 minutes to infuse the mint flavor.

While the mint steeps, whisk together the granulated sugar, egg yolks, and salt in a medium bowl until light and creamy.

Once steeped, strain the cream mixture through a fine-mesh sieve into another bowl, pressing on the mint leaves to extract all liquid. Discard the mint leaves. Reheat the strained cream mixture over medium heat until hot, but not boiling.

Slowly temper the egg yolk mixture: gradually pour about half of the hot cream mixture into the egg yolks, whisking constantly to prevent the eggs from scrambling. Pour the tempered egg mixture back into the saucepan with the remaining hot cream.

Cook over low heat, stirring constantly with a spatula, until the custard thickens enough to coat the back of a spoon (about 5-8 minutes). Do not boil. The temperature should reach 170-175°F (77-79°C).

Remove from heat and stir in the vanilla extract and optional green food coloring. Pour the custard through the fine-mesh sieve again into a clean bowl, nestled over a larger bowl filled with ice water (an ice bath). Stir occasionally until the custard is completely cooled. This helps stop the cooking process and creates a smoother texture.

Cover the cooled custard and refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled (40°F / 4°C or below).

Pour the chilled custard into your ice cream maker and churn according to the manufacturer's instructions, typically 20-30 minutes, until it reaches a soft-serve consistency.

Transfer the churned ice cream to an airtight freezer-safe container and freeze for at least 2-4 hours, or until firm.

In a blender, combine the cubed watermelon, granulated sugar, and fresh lime juice. Blend until completely smooth. Taste and adjust sugar if needed.

Pour the watermelon mixture into a shallow freezer-safe dish (such as a 9x13 inch baking dish). Place in the freezer.

After 30-45 minutes, use a fork to scrape the frozen edges of the granita towards the center. Return to the freezer.

Continue scraping with a fork every 30-45 minutes for about 3-4 hours, or until the entire mixture is frozen solid and forms light, icy flakes.

Scoop the firm Mint Ice Cream into serving bowls. Top generously with the flaky Watermelon Granita. Garnish with a small fresh mint sprig, if desired. Serve immediately.