Preheat the oven to 425° F. Chop off the top ¼ inch of the garlic heads, exposing each of the cloves. Wrap each one separately in tin foil, leaving the top open.
Put a teaspoon of olive oil on each garlic head (on the side that was cut open). Close off the tin foil, and place in the oven on the rack, right-side up.
Bake for 45-50 minutes, or until the garlic heads are soft and the top is caramelized. Open the tin foil (carefully), and allow the garlic to cool for 30 minutes.
Squeeze the softened garlic out of the cloves and into a large bowl. Use a fork to mash the soft garlic. Then add the softened butter, parsley, rosemary and thyme (if desired). Mix well.
Taste test to determine of the additional salt is necessary--I only sometimes add this because brands of salted butter have varying quantities of salt content. Then mix well.