Preheat the oven to 400°F with a rack in the center position.

Slice off the tops of the garlic and drizzle with the olive oil. Wrap the garlic in foil and place, seam side up, on the center rack. Roast until caramelized and tender, about 40 minutes.

Fill a large pot with water and add the potatoes, bay leaves, and 3 teaspoons of the salt. Bring to a boil over high heat, then reduce the heat to low and simmer until the potatoes are fork-tender, about 15 minutes. Drain.

Return the potatoes to the pot. Discard the bay leaves.

Using a potato ricer or masher, mash the potatoes. Once the garlic has cooled enough to handle, gently squeeze the roasted garlic cloves into the pot with the mashed potatoes and mash to combine.

Place the butter, milk, and remaining salt in a small saucepan. Cook over medium-low heat until the butter has melted and the mixture is hot, but not boiling. Pour the mixture into the potatoes, stirring quickly with a large spoon until fully absorbed. Season with salt and pepper to taste.