In a medium pot over medium heat, combine the Coconut Milk (1 can), Cream of Coconut (1/2 cup), Fresh Ginger (1/2 inch), Granulated Sugar (1 cup), and Fresh Lemongrass Paste (1 tablespoon).
Bring to a gentle simmer and cook, stirring frequently, until the sugar has completely dissolved, 3 to 4 minutes.
Remove from the heat, stir in the Buttermilk (2 cup) and Vanilla Extract (2 teaspoon), and allow the mixture to steep for 20 minutes.
Strain into a freezer-safe, 9x13-inch (23x33cm) baking dish or pan, discarding the solids. Allow the mixture to come to room temperature, cover, and refrigerate for at least 8 hours.
Transfer the dish to the freezer and freeze for 1 1/2 hours. Remove the granita from the freezer and scrape with a fork, breaking it up and flaking it until you have a fluffy frozen slushie. It will likely be quite loose still, but that's okay. (The scraping helps create ice crystals, which give the finished dessert its requisite crunchy, flakey texture, and it keeps the mixture from freezing solid.)
Return the dish to the freezer for 1 hour, and scrape again. Return to the freezer for 1 more hour, and then give it a final scrape before serving.
Meanwhile, if you plan to serve the granita with the coconut flakes, put them in a small dry skillet over medium heat. Toast the Unsweetened Coconut Flakes (to taste), stirring occasionally, until it has browned very lightly and smells slightly nutty, 3 to 4 minutes. Keep an eye on it, as coconut goes from toasted to burnt very quickly.
Serve the granita with the Cantaloupe (1) and toasted coconut, if desired. Leftover granita will keep, in a covered container in the freezer, for up to 6 months. If completely frozen, let it warm up for 30 minutes at room temperature before giving it a final scrape.