Halve the vanilla pods lengthways, scrape out the seeds, then add to a saucepan with the pods. Add the sugar, and the zest and juice from your lemon and lime.Bring to the boil, then simmer for a couple of minutes until all the sugar dissolves.Take off the heat, pour into a bowl and leave to cool. Don’t put your finger in for a taste, though, as it could burn you.Once the syrup is cool, remove the vanilla pods.Peel, destone and scoop the avocado flesh into a liquidiser with the syrup and milk. Blitz to a smooth, light consistency – like a smoothie.If you’ve got an ice-cream maker, put your mixture into it and churn till smooth and frozen. If not, pour it into a large baking dish or deep tray and pop it in the freezer, giving it a whisk every half hour or so until frozen and smooth.You can eat it right away or transfer it to little containers, cover and pop in the freezer for later.