Place a 1 litre shallow baking dish into the freezer to chill.
Dissolve the caster sugar into the hot coffee over low heat and allow to cool.
Pour the coffee into the chilled dish and freeze for 40 minutes.
Gently scrape ice crystals from the edges into the center using a fork.
Repeat the scraping process every 20 minutes until the mixture is entirely crystallized.
Transfer the crystals to a lidded box and store in the freezer.
Whip the cream, vanilla seeds, and icing sugar together until reaching a dropping consistency.
Serve the granita in chilled glasses topped with the whipped cream.