1. Place garlic in a large pot of salted water and bring to a boil over high heat. Separate cauliflower into small florets and add to pot. Bring back to a boil, reduce heat to medium-high and cook until cauliflower is very tender, 10 to 12 minutes. Drain well, transfer back to pot and cook over medium-high heat, stirring constantly, until dry, about 3 minutes.
2. Combine garlic, cauliflower, olive oil and buttermilk in a large bowl. Use a potato masher to mash cauliflower until thick and pulpy. Transfer to a large saucepan and bring to a simmer over medium-high heat. Cook until warmed through, about 5 minutes. Season with salt and pepper. Sprinkle with parsley, if desired, and serve hot.