I looked up some recipes for your Catalan prawns and I found one similar to the pil pil recipe on the website food.com (GAMBAS AL AJILLO (SHRIMP W/ GARLIC) CATALONIA). Some other recipes over-egged things a bit too much but that one looks good. Not sure that you need butter, but hey, when is butter ever bad?   One difference is that, in the three places in Andalucía I had the dish, the garlic was not minced but was cut into quite large pieces, maybe each clove cut into about four or five, and plenty of it. We almost wrestled each other to get at those lovely, sweet chunks of garlic. I've said it before and I'll say it again: throw your garlic crusher in the bin! All you need is a sharp little knife. I have one of those cheapie silicone tubes which you put a clove into and roll on the worktop - and out comes a perfectly peeled clove!