Place the avocado and lime juice in a blender and whizz up until it's a smooth puree.
Put the avocado-lime mix in a food processor with a whisk attachment (or use a handheld whisk) and add the double cream, caster sugar, and vanilla extract.
Whisk on a medium-high setting for about 3 minutes.
Place in a freezable container for 3-4 hrs.
(Any longer and it's a bit hard to scoop out.
This isn't in itself a detriment to the recipe - just remember to let it thaw slightly before serving for a creamier `soft scoop' effect).