Cook 2 tablespoons butter and 2 tablespoons all-purpose flour in a heavy-bottomed 4-quart pot over medium heat, while stirring, until the flour smells toasty, about 3 minutes. Whisk in 2 cups milk or chicken broth, bring to a low simmer, and whisk until smooth. Whisk in 2 more cups milk or broth and the florets from one 2-pound head of cauliflower. Cover the pot and simmer gently over low heat until the florets fall apart when you crush them against the inside of the pot with the back of a spoon, after about 25 minutes. Puree the soup in a blender or with an immersion blender. If you wish, you may also work it through a food mill or strainer with the back of a ladle, into a clean pot. If you want it perfectly smooth, work it again through a fine-mesh strainer. If you wish, add ¼ to 1 cup heavy cream and bring back to the simmer. Thin if necessary with extra milk or broth. Season to taste with salt and white pepper. Serve immediately in hot bowls. Makes 6 servings without the cream or added liquid.
