I had some leftover parsley in the fridge that I wanted to use up - and I had heard of this concept. I was actually a bit skeptical about the butter, but it worked out ok. I just whirled everything in the blender. The kids were licking the spatula. I think I prefer "real" pesto, but whenever I need to use up parsley I will use this recipe. BTW, I discovered I had no marjoram, so I left it out.
I made up a batch and I liked it, thought I decided it needed a bit of salt and lemon juice. I spooned it atop cod filets while they were sauteing -- I'm sure the butter helped make it lovely! I also used it with a little more olive oil to dress a spinach/sun-dried tomato/ mozzarella/ salad with the fish. Nice dinner!
to be fair, i am giving this 5* instead of 4* because there is always the temptation to compare this to the regular pesto - and of course it won't quite measure up if you opt to go for parsley. but this is still a very good "poor man's pesto", since it's very inexpensive - where i live basil is heaps more expensive than parsley. i did omit the parmesan but used the walnuts and we enjoyed it with salmon.