Pre heat oven to 200c.
Peel away most of the outer paper layers from the garlic heads (but leave the head intact) then cut approximately 1 cm (½") off the top of each garlic head and discard.
2 whole heads/bulbs of garlic
Drizzle half the olive oil on top of each head and sprinkle with a little salt and pepper.
1 tablespoon olive oil, salt and pepper for sprinkling on garlic to roast
Wrap each head in aluminium foil and place in the oven to cook for 30 minutes.
Once roasted, take them out and open the foil packets to allow the garlic to cool (be careful as steam will escape).
Once cool enough to handle, squeeze the flesh out of the individual cloves into a small bowl and mash until smooth (see my notes).
Combine butter, salt and garlic paste, mixing until smooth (I find a potato masher can be good for this). Taste for seasoning and add more salt as required.
200 g unsalted butter, ¼ teaspoon fine salt
Shape the garlic butter into a log then wrap in plastic wrap or baking paper and refrigerate until needed.