Add cauliflower and garlic to a large pot, preferably a Dutch oven, and add enough chicken stock to cover. Set over high heat and bring to a boil. Cook until cauliflower is fork-tender, about 12 minutes. Drain thoroughly. Return cauliflower to pot. Add butter and blend. Add half the milk or cream and blend. Purée with an immersion blender or in a traditional blender. Add more milk or cream, blending constantly, until desired texture is reached. Season to taste with salt and pepper.