Why not just make it with milk? I'm sure it would be possible with breastmilk (after all, you can make breastmilk ice cream!), but you might waste a lot trying to get it right. Formula isn't really milk, so it might be a bit more tricky (and not taste very nice). But as your LO is over 6 months, I'd just try making your own from cow's milk or goat's milk if you're avoiding cow's milk for any reason.