Note: you will need an ice cream machine to churn the chocolate sorbet base.
Chop up the chocolate (225g) into small pieces (bean sized). Set aside.
Add the water (480g or 2 cups), sugar (150g or ¾ cup), corn syrup (2 tablespoons), and salt(¼ teaspoon) to a pot. If using the cocoa (2 tablespoons), sift it in. Whisk the mixture well and set over medium low heat. Stir as the sugar melts.
When the mixture starts to steam and the sugar is fully dissolved, turn off the heat.
Add the chopped chocolate and stir as it melts. Use an immersion blender to blend the mixture very well (if you don’t have one, transfer the mix to a blender or food processor and blend until smooth).
Set a fine mesh sieve over a heat safe bowl. Pour the base through the sieve (this is just to get rid of any lingering bits that didn’t get blended). Stir in the vanilla. Let the mix come to room temperature then cover and set in the fridge overnight (or up to 2 days) to fully chill.
When ready to churn (follow your machine’s instructions for churning, it may ask you to freeze the bowl for a day beforehand) gently whisk the mix then pour it into the churning bowl. Churn to freeze then transfer to a freezer safe container and freeze. The sorbet will keep for about a month but I find it’s best eaten within the first two weeks.