Place cherries into a large saucepan with 300ml water and the sugar.
Cook gently for 10-15 min., until the cherries have softened completely.
Blend in a food processor, then push through a sieve, taste, and add a little more sugar if desired. Stir in the vanilla essence, if using.
Pour into a large dish, freeze for 1hr. Use a fork of scrape the frozen edges into the middle, freeze again. Repeat this step 2 or 3 times at hourly intervals until granita has set.
Serve in glasses.