I've never done this, so take my words with a grain of salt... just going off what I've heard from browsing through the recipe forum many times.

The more fat that's in the mix the better. Next time try 3 cups of whole milk and leave out the water. Maybe even throw a bit of pure cream or butter in there too. Grind up your herb really well, as close to a powder as you can get it. Mix the herb with the milk, bring it to a boil, then lower the heat and let it simmer for good while (maybe an hour or so), stirring often to keep it from growing skin and adding water as needed to keep too much from evaporating.

That's what I'd do, anyway.