Bring the water, sugar, and cocoa to the boil in a small pot until the sugar has dissolved. Allow the mix to cool to room temperature.
Add the coconut milk and stir to combine thoroughly.
Pour cooled mixture into a large ziplock bag. Place the bag on a tray, flatten it out, and seal. Freeze for at least 4 hours or overnight.
Once frozen, remove from the bag and break into chunks. Place in the bowl of a food processor and whizz for 30 seconds to 1 minute until smooth and creamy.
Scrape down the bowl with a spatula and transfer to a pre-frozen metal container. Smooth the surface and freeze until solid, about 2-3 hours.
Remove from freezer 5-10 minutes before serving to soften slightly.