I quit roasting garlic in the oven about 20 years ago when I bought a Garlic Express
. Mine looks just like this one and it roasts up to 3 heads of garlic perfectly. The garlic pops out of the papery skin just like butter. I roast a lot of garlic to incorporate into bread dough.
I always thought heating the oven, especially in the summer, just to roast a few heads of garlic to be inefficient. The little roaster does it in far less time and shuts off automatically so the garlic isn't overdone.
I didn't pay much for the one we have, perhaps $25 USD and I was surprised to see how they've gone up when I searched on Amazon a few minutes ago. I imagine they could also be found on eBay for a reasonable price.
I just couldn't rationalize the energy and time it took to, first, preheat the oven, then to cook the garlic. Just didn't make sense to me.
As for making the butter you speak of, I can offer no advice except for the cooking method for the garlic.