This is the simplest, least concocted option I've got.
For the right consistency, make sure your avocado is nice and ripe. You can tell by squeezing it gently (not too hard or it will bruise). If it gives, it's ripe. The more it gives, the riper it is. There is a limit to this - ripe can quickly become rotten, so keep an eye on your avocados. I keep mine on the counter until they're ripe, and then if I'm not ready to eat them, I stick them in the fridge to keep them from going bad in, like, a day. Also, if you're purchasing Hass avocados, the small, pear-shaped, pebbly skinned variety that most grocery stores carry, the skin darkens from green to near black as the avocado ripens.
Then all you have to do is open it up and scoup out some of the flesh and spread. Sprinkle with salt and there you are! Green butter! Amazing!
Avacados are also similar to marmite in their vegan-friendly healthinesses. They are relatively high in fat, but it's the monosaturated kind - the good kind - and have apparently been shown to reduce bad cholesterol and increase good cholesterol (which we all know isn't just neutral but actually good for us) when eaten regularly. Among other things, they are high in B vitamins, potassiam and fiber. And they kind of look like dinosaur eggs.