Take off the outer leaves of the cauliflower and trim the bottom off of the stem.

Using a paring knife, cut around core to remove it completely; thinly slice core and reserve.

Cut a heaping 1 cup of 1/2-inch florets from head of cauliflower; and set aside.

Cut remaining cauliflower crosswise into 1/2-inch thick slices.

Melt 3 tablespoons of butter in large saucepan over medium-low heat.

Add the leek, onion, and 1 1/2 teaspoons salt; cook, stirring frequently, until leek and onion are softened but not browned, about 7 minutes.

Increase the heat to medium-high; add 4 1/2 cups water, sliced core, and half of sliced cauliflower; and bring to simmer.

Reduce heat to medium-low and simmer gently for 15 minutes.

Add remaining sliced cauliflower, return to a simmer, and continue to cook until cauliflower is tender and crumbles easily, about 15 to 20 minutes longer.

Meanwhile, melt remaining 5 tablespoons butter in 8-inch skillet over medium heat.

Add reserved florets in the measuring cup and cook, stirring frequently, until florets are golden brown and butter is browned and has a slightly nutty aroma, 6 to 8 minutes.

Remove skillet from heat and use slotted spoon to transfer florets to small bowl, and reserve the browned butter in another bowl.

Toss florets with vinegar and season with salt to taste.

Process the soup in blender until smooth, about 45 seconds.

Rinse out pan.

Return pureed soup to pan and return to simmer over medium heat (soup should have thick, velvety texture but should be thin enough to settle with flat surface after being stirred) and seasoning with salt to taste.

Serve, garnishing individual bowls with browned florets, drizzle of browned butter, and chives and seasoning with pepper to taste.

Store remaining soup in an airtight container in the fridge or allow to cool completely and seal in a ziploc in the freezer for up to 1 month.