In a mixing bowl or large measuring cup, whisk together the coffee, granular sweetener, and rum, if using, until well combined.

Pour the mixture into a large shallow dish, such as a 9x13 inch glass or ceramic pan. Place the dish in the freezer.

After 45 minutes, remove the dish from the freezer and use a fork to scrape and break up any ice crystals that have formed along the edges of the pan. Return to the freezer.

Repeat this scraping process every 30 to 40 minutes for at least 3-4 more times, using the fork to mix the ice crystals and breaking up any large icy clumps until you achieve a fine, icy granita consistency.

In a separate medium bowl, whip the heavy cream with the powdered sweetener and vanilla extract until soft peaks form.

Serve the granita immediately in small dessert dishes, topping each serving with a dollop of the whipped cream. Optionally, sprinkle with cinnamon.