Melt the butter in a pan on medium heat. Fry the cocoyam in the butter stirring continuously until they start to look translucent. Add the double cream, coconut milk, sugar and salt and simmer for about 10 - 15 minutes until the cocoyams are cooked.
Transfer to a blender and blend to a smooth paste. Allow to cool to room temperature. Transfer to freezer-safe bowl, cover with a plastic wrap and freeze for 2 hours.
Remove from freezer and beat with hand mixer to break up the ice crystals forming, then return to freezer for another 2 hours. Remove from freezer, and beat again with hand mixer.